The Decor: Mix and match different colored (and patterned) plates, napkins and party hats. Lemon yellow, lime green, sky blue and cherry red make a cute, cheery combo -- or go with your little guy's favorite colors. Decorate your space with plush toys (like Ugly Dolls) and balloons.
The Cake: Create a killer cake like this one using pre-dyed fondant decorated with googly eyes and topped with fondant monsters. (Scared of fondant? Don't worry -- it's a monster theme, so it won't matter if your creatures look a little lumpy and lopsided!) Or, dip cake pops in colored candy melts and add on googly eyes (the more, the kookier!).
The Activity: Ayars created a monster photo booth complete with cardboard "monster mouths" on sticks for guests to hold up to their faces, but you can create a fun (or should we say frightening?) photo op simply by encouraging your little guests to make their scariest monster faces and snapping pics; then print them out and stick in them colorful foam frames.