There are many blue coins that cannot be found with specific levels, they are hidden around Delfino Plaza itself.
Fruit ladies (x4)
Banana lady
Look for a Pianta lady standing near a wall just south of the central dock. Talk to her and she offers you work collecting fruit for her. Gather three bunches of bananas and place them in her basket, then talk to her again to be rewarded with a blue coin. Bananas can be found in one of the fruit stalls, on a rooftop in the southeast corner of the square, and in the top of a tree west of the Pianta statue. Can be repeated for a 1-up.
Coconut lady
Go to the boathouse in Delfino Plaza (where the portal to Ricco Harbor is) and look out to sea and you'll notice a small island with a palm tree and a Pianta lady. Swim out there and talk to the lady. She'll ask you for 3 coconuts, and as luck would have it, there are 3 coconuts hanging in the tree right next to her. Spray them to knock them down and then put the in her basket one by one. Don't worry if some of them fall in the sea as they will grow back again straight away. Can be repeated for a 1-up.
Pineapple lady
This woman is standing next to the giant Shine Gate north of the central dock area and she asks for pineapples. Her basket is high off the ground so you'll need to throw (jump + B) the fruits in. Finding the best place to throw from takes a bit of trial and error but isn't as hard as it seems. Alternatively, you can bounce on her head three times and if you land on the edge of the basket, you can then simply drop the fruit in.
Durian lady
This is by far the toughest fruit lady challenge. Look for the Pianta lady to the left of the shine gate. She asks for durians and they are the spiky fruits that Mario can't carry, only kick around. So it's football time.
There are a few ways to get the durians to the basket but this seems to be the simplest. Get a durian from under the awning near the boathouse. You can also get durians from the fruit market but it's farther away. Gently kick it north along the boardwalk, then when you get close to the end aim toward the tree in the distance (this is shown in the picture on the right). If you run at the durian and press 
just before you get to it, you will give it a mighty kick that will send it far into the distance, then hover over and see where it landed. Hopefully it didn't bounce into the water (if it did, just keep trying), so from here kick the durian through the archway and then go left to the little hill.
Getting the fruit up the hill can be very tricky because of the camera angles in such a tight space. You need to place the fruit at the bottom of the hill, stand directly behind it and the spin the camera around so it is facing Mario. Now run in a dead straight line up the hill with enough speed to get the Durian up there. Once you get up the hill, kick or spray the fruit into the basket. Now just do that two more times to get a blue coin as a reward. Alternatively you can repeatedly run and press to dive to push the Durian up the hill quickly. Be careful that doesn't go over the wall and into the water under the shine gate!
Graffiti (x5)
Shine gate M
Locate the large crate in front of the Shine Gate (on the right side). Ground-pound this crate to smash it and reveal a graffiti "M", then spray the wall clean for the coin to pop out.
Rooftop M
Locate the large crate on the rooftop over the inlet between the central dock and the ocean. Ground-pound this crate to smash it and reveal a graffiti "M", then spray the wall clean for the coin to pop out.
Tower M
Go to the tower in the northwest corner of the plaza, near the red cannon that fires you over to Pinna Park. Look on the north side about about half way up to find a graffiti "M", then clean it off for the coin to appear.
Paired crosses (x2)
Locate a graffiti cross on the east side of the base of the Pianta statue (right of the Bianco Hills entrance). Locate a corresponding cross one the west side of the southwest tower (the one with the radio dish). Wash one cross off to make a blue coin appear at the other cross, then quickly get to the other cross to collect the coin before it disappears. The slip-slide move is very useful here for covering the distance in time, or wait until you get the turbo nozzle and use it. Do both crosses to collect two coins.
Miscellaneous washing (x3)
Flaming Pianta
This might sound like the name of a refreshing cocktail, but after you open Gelato beach there actually is a Pianta fellow running around wearing a backpack that's on fire! He normally is found on the beach near the lighthouse but may also appear next to the central dock. You need to chase him around with FLUDD going to put out the fire, then talk to him to be rewarded with a blue coin. After collecting the coin, the same Pianta can be seen wandering around with his backpack full of red mushrooms.
Blue parrots (x2)
There are 2 blue parrots flying around Delfino Plaza which turn in to blue coins when you spray them. They fly around but if left alone they perch in the following locations:
- On the roof of the building to the left of the shine gate.
- On the roof of the building north of the Pianta statue.
The blue birds usually hang around with some green birds, and are very wary of Mario's presence so you'll have to squirt them from quite far away if you want to catch them sitting still. You can also wait where they would normally land and squirt them in mid air. When you squirt them they fly away for a bit and then come back to the same spot. A few good hits should be enough to turn them into coins.
Yoshi needed (x2)
Northeast tower window
Go to the rooftops in the northeast section of the plaza, between the shine gate and the lighthouse. Look on the north side to find some orange goop covering a window. Get Yoshi and spray the goop with fruit juice and it melts away. You can now hop into the window and collect the coin there. When you re-enter the plaza the goop will be restored even though there is no coin behind it.
Northwest tower window
Go to the rooftops in the northwest section of the plaza, near the red cannon, and look on the east side of the tower to find some orange goop covering a window near the top. Get Yoshi and spray the goop with fruit juice so it melts away. Yoshi's fruit juice range is pretty good so you can to this from the next building over. You can now do one of the following:
- Abandon Yoshi and climb the small ledge circling the tower. The sidestep move may come in handy here.
- More difficult, do a spin jump and float with Yoshi all the way up there.
Collect the coin when you reach the window.
Stone pillars in the sea
Near the tower on the east beach, where the portal to Gelato Beach is, there are some stone pillars a little way out to sea - you've probably been there before by this point. One of the further-out pillars has a double door in the bottom of it, which hides a blue coin. Use FLUDD's Turbo Nozzle (if you haven't unlocked it yet, you'll have to wait a while) to skim across the ocean and smash through the double doors and grab the coin.
List of all 20
This should be integrated and expanded upon like the above coins.
- Go to Delfino Airstrip if you have unlocked it. Get the Turbo Nozzle and go through the door (yes, you break it). You should see a block of ice. With the Squirt Nozzle, spray it until it melts. Collect the coin and exit the area.
- The two corresponding X marks. One is on the statue leading to Bianco Hills and the other is near the Pianta who takes you to the Airstrip.
- Same as above.
- M Graffiti behind a crate.
- There is a Pianta on fire! Spray him until the fire is out and talk to him.
- There is a lady Pianta on the island. Talk to her. She wants you to put three coconuts in the basket. There is a coconut tree right next to her, so I don't know why she can't get it herself. Spray down the coconuts, put three in her basket and then talk to her.
- There is a Pianta lady near the Shine Gate. She wants three pineapples. Get the pineapples from the shop and put/throw it in her basket.
- There is a Pianta lady across the water. Talk to her and she wants three durians. Go to the shop. The catch is that you can't carry the durians, you can only kick it. Kick it above the wall to the next level, then kick it across the water. Put the durians in her basket.
- There is a Pianta lady across from Pianta lady from number 6. She wants three bananas. Put three bananas in the basket (if you didn't notice, she said three bananas. There are three bananas in one bunch, So technically, she wants NINE bananas).
- There is a M Graffiti behind the crate near the Shine Gate.
- There is a M Graffiti behind a building on the wall.
- Get Yoshi. There is a force field on the top of a building near the Pianta on the roof,
- Same as 11, but the force field is on a building near the lighthouse.
- There is a blue bird on the roof near the statue.
- There is a blue bird across from the force field in 12. Go across all the roofs to find it.
- Get the Turbo Nozzle. On one of the pillars, there is a small door. Go through it for a blue coin.
- In the jail cell under the bell tower nearer to the beach.
- In the sewers near Corona Mountain.
- In the sewers near the center of Delfino Plaza.
- In the sewers beneath the ocean.