K.L.
Have you considered having the party at a playground? There is a tot-lot in Rogers Forge (I see you live in Towson) that is awesome, but there are plenty of other playgrounds and parks around that would provide lots of fun for the kids.
When is the party going to be? If it is warm enough outside, set up a sprinker and let the kids run around in the sprinkler. Bubbles, ride-on toys, balls and all those other kid toys will keep them occupied.
You don't need to entertain the parents, the party is for the kids. I think it is crazy how much money parents spend on birthday parties now! I had my parties at my house with my friends and family and everyone had a blast. Maybe times were just simpler back then (I am 25, so it really wasn't that L. ago).