What are some other hidden places?

Hint 1 of 4:

(1) Once you get into the firehouse, check out the boxes out of the window.

Hint 2 of 4:

There are some items on top of there.

Hint 3 of 4:

(2) Look behind the curtains by the radio show.

Hint 4 of 4:

(3) Jump on top of the fire truck for some power-ups.