When I had a Pokemon birthday party, twelve or thirteen years ago (has it really been around that long?!), we had a scavenger hunt of sorts where we "rescued" Pikachu from Team Rocket at the end.
Basically, we had different activites, like having to pop balloons by sitting on them(my mom somehow tied every activity in to pokemon, and it was like having different steps to beating Team Rocket). After every activity was done, we got a clue that led us to where the next activity was set up. The final clue led us to where Team Rocket had 'hidden' Pikachu (we used a stuffed Pikachu that we had, and if I remember correctly, there might have been a Pokemon card or two for each of us there as well). I remember that all my friends loved it, and it was the talk of my class for weeks afterwards.