What are some fun "missions" to do at home or around the neighborhood?

One sort of scavenger hunt you can do with eachother without taking the fun out of it for yourself is to do a disposable camera hunt. Get together and pick a few items to hunt down and then spread out maybe in teams of two to make it more fun, and take pictures. Then rendez-vous at a one hour photo place put your photos in, and go get ice cream or something together while you wait. Then you can compare photos and vote on which one is the winner in each categorey. You could do a similar thing with digital cameras if you wanted. Make the items obscure like "a person with a moustache" or "a photo of a 'no photographs' sign" so that it makes it both difficult and "spy-like" to try and take a covert photo. Just be, you know, smart about it (ie. stay away from playgrounds). If you can't find the item you can make a rule where the person who comes closest (or whatever criteria you choose) wins.

Pick a victim or villain (someone ant home or in neighborhood). Spy on them. Make homemade weapons or devices (paper, cardboard, aluminum foil, etc.) Dress in black or something like that. And when you feel like it, pretend like you are battling the enemy and execute :P ENJOY

Buy a water balloon launcher....fun for hours.

oh and fill the balloons up with piss

have race with ur cousins.