Using old crap from around the hous and free junk from garage sales is fun. We have a couple car doors and dolls. I would really like to find some mannequins but dont know where. If its colder out, ice is really fun too, freeze up some 1gal ice cream buckets, take the chunk of ice out and have a blast, they usually only take one shot from anything >.22 but the explode pretty nicely. The R/C car idea and balloons sounds like a lot of fun, unless you are with people who are bad shots then it only lasts a few minutes.
Old furniture can be a good time, fun to see how long it takes to "saw" the legs off.