you might want to look into renting a room at a community center, church, or synagogue and have things like hula hoops, balloons, bean bag toss and/or ring toss, pop-up play tents or tunnels for the kids to play with. If there will be younger kids in attendance too, one thing our local parents of multiples club did at our recent holiday party (held at a church gym) was to fill a large wading pool with plastic "ball pit" balls and surround it with one of those kiddie-play yard enclosures and it was a huge hit with the toddlers (and a few slightly-older-than-toddlers who snuck in with their siblings). Another friend whose birthday-party-at-the-park got rained out moved their son's birthday party to their church's social hall and one of the big hits was the balloons - just air-filled so they were sitting on the ground, but the kids loved to pick them up and bat them back and forth, and since the balloons move so much more slowly than a ball, no chance of anyone getting hit by one or anything knocked over, just an occasional startle when a balloon popped.