Have a truck pull a flatbed trailer. You can decorate it or just wear costumes. Some people can walk and others can ride. Then trade off. It gets really hot, so dress lightly.
My german club in Jr High got sponsors for their float and put ad banners down the side to pay for a special event.
Interactive floats do well in small slow moving parades. If you have something to do with the audience, it creates interest.
It doesn't really matter what you do - you could toss out candy or squirt *willing* onlookers with water guns. One gag is to have a water gun fight between people on the float then at the high point have some one grab a bucket (filled with confetti) and start chasing people. Then they "accidently" dump it on the audience. Kids love that one.
