We didn't have a party, but for our reveal we made cupcakes. Covered them in icing so you couldn't tell what they were and decorated them with question marks, B ' s, G's and other baby related things. Then we took a picture of all the cupcakes, a picture of me holding a B and my husband holding a G, and then a picture of us holding the cupcakes up to reveal the blue cake.
For a party you could have a cake that you need to slice into to reveal, or a box full of one color balloons, or a scavenger hunt leading to a piece of paper that says girl or boy.
We're having one. Pinterest does have great ideas. Etsy has good invitations as well. I'm going to do the balloon reveal. We find out the sex March 30th!