Even though I take my bowling pretty serious, me and my sister when we go practicing usually do the last game "fun" where we make up silly rules to play. I would do something like that. Definitely bring little prizes to give out also to the winners, and let them choose the next contest. You can do it by game or frame.

some ideas to get you started:
- bowling with eyes closed
- using 2 balls (1 in each hand)
- bowling lefty (or righty if you are a lefty)
- bowling between legs
- bowling backwards (walk backwards on lane)
- have a few people stand just before the foul line with legs spread apart, bowl between their legs
- spin around several times before (or even while) bowling. Similiar to the dizzy bat effect, haha

Also give out prizes for strikes/spares, etc.

Just make sure you keep it safe and have fun. Don't do anything stupid that will get you kicked out.