I don't know what the weather is like where you are, but here a popular idea is bringing in a food truck and having an outdoor meal. of course, that only works in July and August unless you set up a big tent. Look for interactive or unusual entertainment, such as caricature artist, living statues, chainsaw sculpture, dueling pianos.
Most important, avoid hiring strippers or anything close to it, and also avoid clowns.
Microsoft Regrets Hiring Scantily Clad Dancers For GDC Party | HotHardware