I remember freeones has a forum with a point system. You can use those points to get videos or a t-shirt.
Have you tried any of the virtual sex worlds? Red light center has their own virtual currency, called Rays. You can buy stuff with them. Just logging in gets you some rays and casino chips. I don't know how popular it is though.
http://www.redlightcenter.com/?trq=120084
Good idea though. You can have raffles for gift cards and stuff to cut down on prize costs.