A side-quest in Traverse Town involves finding postcards scattered around the world for prizes.
Postcard 1
The first postcard can easily be found in the first district. Go past the Accessory Shop to find an area with a large blue safe in the corner. Examine the safe for your first postcard.
Postcard 2
From where you got the first postcard, turn around and jump onto the roof of the Accessory Shop. Walk to the other side of the roof (facing the entrance of Traverse Town) and jump onto the ledge on the front of the Accessory Shop. From the ledge, jump to the other side of the roof and look behind the chimney, where a treasure chest is hiding. Open it for your second postcard. Though High Jump or Glide would make getting this postcard easier, they aren't necessary.
Postcard 3
The third postcard can also be found in the first district, this time in the Item Shop. Go up the staircase and you'll be able to see a ceiling fan. Lock on to it, jump, and hit it with your keyblade for the third postcard.
Postcard 4
When entering Traverse Town, go to the left where you'll see what looks like an outdoor restaurant. There should be a clearly visible Blue Trinity. Use it and you'll end up on a balcony above the tables. Open the treasure chest there for the fourth postcard.
Postcard 5
This postcard is in the second district. You'll be able to find it above the Boots & Shoes shop. From where you enter the district, jump on the lamp to the right and then jump to the right corner. You'll land on an awning with a treasure chest on top of it. Open it for the fifth postcard.
Postcards 6 & 7
Postcards 6 & 7 are obtained together but require two steps. First, go to the third district. Use lightning on the switch with the lightning symbol on it. After this step, go back to the second district and enter the Gizmo Shop. Step on every raised platform and then examine the clock to obtain two postcards.
Postcard 8
For the eighth postcard, you'll start out in the second district. Go to the right of the Gizmo Shop and you'll see a ladder. Climb the ladder and follow the path on the left of the roof to another roof. Use High Jump or Glide to get to the next roof. Keep going straight ahead on that roof until you hit a passage to the third district. Go through it and you'll end up on a third district balcony. Go to the right of the balcony and examine the corner for Postcard 8.
Postcard 9
The ninth postcard can be found back in the first district in the Accessory Shop. Use the Green Trinity here to open a path to the Synthesis Shop upstairs. Examine the paper on the wall next to the fireplace and you'll get Postcard 9.
Postcard 10
After getting out of Monstro, Geppetto will be available to visit in the first district of Traverse Town. Go to his house and examine the shelf in the corner. You will then receive your final postcard. After collecting every postcard, go to the mailbox in the first district to collect your prizes.