All I can come up with is a "Pin the Snake on Medusa" game ... but I don't know much about the book series - just seen the film. However, there were a few blogs that offered up some good suggestions:
1. The invitations - make them look like a scroll and tie them closed with a ribbon. You can make the paper look older by staining it with coffee and burning the edges.
2. Make a Sign to put over your door called "Camp Half Blood." If you choose to have competitions as they did at the camp in the book then make up some gold, silver and bronze medals.
3. Have swords for them and let them make up a shield out of foam board and decorate it.
4. Have a treasure hunt. One lady make a treasure hunt with clues leading the children at one point to a dart board with balloons they had to pop to get the clue and then finally to a sword that they would use to kill the Minotaur. This was really a durable sword or pinata stick and the Minotaur was an altered cow pinata - easy to make it look a little more evil. I'm going to suggest that if you do the clues thing - write some of them in Greek and give them a guide to "decode" it. =)
5. She also recommended a blue cake as in the book Percy's mom feeds him blue food on special occasions.
One blog:
http://www.make-it-do.com/celebrate-it/percy-jackson-and-the-olympians-party/
You may want to check out Oriental Traders. They have 72 lightning tattoos for $4.99, a dozen Torch Award medals for $3.99, they have a decorate your own sword kit 12 for $12.99 but they are not good for play or inflatable swords for $9.99/dozen. They have many varieties of snakes on there too if you want to do something else related to Medusa and I'm sure if you poke around you could find some interesting party favors. I Google discount codes when I purchase there and I can usually find free shipping and/or an additional percentage off.
