Would one of your local vets do a 'special price' micro-chipping for the event?
In the musical chairs game replace the chairs with paper plates and the folks have to get their dogs to sit on the plate!
Have a 'dress your dog' race. Gather up a bunch of varying sizes of old socks, shirts and party hats, boxer shorts too if you're really evil. Do a fairly close match up of the contestants and clothes and put piles of the outfits at the far end of the course. At the signal, contestants run to the clothes, dress their dog and then run back. First one back with all their clothes on wins. Here is a picture of Raemi's sister, when they were puppies, at a picnic where we played this game.
If you have a local radio station, why not see if they will put in a 'plug' for you, too? Sounds like you will have a really great time. Have fun! Take and post lots of pix!!