Level 1-2
The first and only chest for this level is in plain site on top of the ledge about halfway through the level.
Level 1-3
The first chest you will need the animal ability to get. Just dig through the dirt to get to it. The second chest is impossible not to get. It's inbetween the two ledges.
SQUEAK SQUAD: Spinni
First time you grab large chest, the whole squad appears. You see Spinni snicker, then battle time. (Worry not, the only one you fight is Spinni.) You can easily run away, you only put up with a few yellow squeaks, but you get NINJA from this guy! Also, take time to enter the lair. It has items.
Level 1-4
The first chest is in the room with the star blocks on the ceiling. Just break them and climb through. The second chest is after the lollipop in-between the two rows of spikes. The third chest is right above your head after the lollipop room.
SQUEAK SQUAD: Storo
You have to beat him, because the screen won't scroll till he's out. Just whale on him. Fire ability is nearby in a bubble.
Level 1-5
Just fly all the way to the top of the area with all the barrels to get the only chest of the level. However, you might wanna check out other barrels, because you get TWO EXTRA LIVES HERE!
Level 1-Extra
The first chest is in the room with the hi-jump enemy. Get the hi-jump ability and fly up the room until you get to a split. Go up the left side of the room and go inside the door to find a treasure chest. The second chest is on a floating platform. Its not hard to get at all.
SQUEAK SQUAD: Doc
Doc will fly around and attack you. His bombs give you Bomb Ability. This is an open space, so take doc out quick. Remember to eat that UFO if he drops it; otherwise, you may still want the laser blaster.