Our local no kill shelter has a yearly bark in the park gathering to raise money. They advertise via flyers, radio and billboards. I believe the cost involved to advertised is minimal to the amount they bring in. Last year they said there were a couple hundred dogs involved. They had doggie games such as limbo, water games and other fun things. It was more like a doggie carnival!

Hope you come up with something. I personally love our dog park!