In Downtown Whoville, you don't need the costume to get past the sleeping policeman just tiptoe past him and the cloak will be inside for future visits.

Don't be concerned if you can't accomplish all missions per level the game isn't linear, so in most cases you'll have to visit other levels first to construct gadgets.

In Who Dump, shoot the birds with the Rotten Egg Launcher and they'll drop food.

To open the safe in the Town Hall, shoot it with the Rotten Egg Launcher it contains a blueprint for the Grinch Copter.

Also in the Town Hall, on the bottom level, you'll find that some of the rugs on the wall look different than the others;