Here it is, my BIG secret:
DECIDE ON PARTY THEME AT LEAST SIX MONTHS IN ADVANCE.
((ducking for cover))
I know. I know.
The second benefit of knowing early on what your theme will be is that you are able to keep your eyes peeled for themed party supplies and catch them when they are SUPER cheap.
Dollar stores are also full of licensed party supplies, you just have to keep your eyes peeled. Since they have their Halloween costumes in stock, I found the inflatable superhero gear (on the bottom right) that should keep the munchkins occupied while all the guests arrive. The superhero dog tags will go in the favor bags, and the mini superhero figurines can be used as table décor.