Don't tell the kids, but a trampoline is really good exercise and a trampoline party can be great fun, so depending on the age group involved why not plan a party with some trampoline games or contests thrown in? Most trampolines have safety nets around them to keep the smaller kids from bouncing off the thing and making a hard landing on the lawn, if yours doesn’t have the net you should really consider getting one. Not only is the net a safety feature but it's a really good place to hang various party themed items for decoration. How about a Las Vegas theme? You can decorate the net with themed setters that can be found at the local party supplies store and glow bracelets can be hung around the edges or you can give them to the kids to wear when the sun starts to set. You can decide ahead what games and contests you want, for instance prizes can be awarded for the highest jump or the most jumps in a minute, or if the kids are older, the first full flip to land on the feet and not fall over, or the most consecutive flips.
When they get the hang of that try doing it with back flips. You can have them play a copy cat game where the first kid does a trick and gets off followed by the second kid who does that trick then one of his own, the third kid does the previous tricks and adds another, the first one to not remember the sequence is out and the last one standing is the winner. The winner gets a token to redeem a prize at the "cashiers" window. This is all fun but very tiring so have plenty of shade, a bucket of virgin Slush Puppy on hand and while the kids aren't actively on the trampoline, they can be having a cake and ice cream party.