I dont know how many your bringing and/or all of your ages... but their are many things my family has done in the past. Like others have mentioned... the parties are a must, and the little boats are also a blast. I have to mention the tours too, they are really fun. All of the "behind the scenes" ones are 16+ but there are a couple family ones too that are like a scavenger hunt through the magic kingdom, and stuff.
otherwise some simple ways to spice up the parks cheap might be to hunt for all the hidden mickey's, or any of the other little special things to do in the parks... paint brush hunt on tom sawyer island, try to wake up tink at the gift shop, or finding any of the number of little props that you can play with ("singing in the rain" umbrella at HS), along with many others.
hope these help.