A couple of ideas for you that can be adjusted to suit the riding level and ages of your group.
1. idea #1 You can try some 'tent pegging" ideas - a skill challenge such as a lance through a hoop (use a broom handle as a lance if you like) you can also try to knock small balls from the tops of traffic cones while maneuvering through an abstacle course. its all about using your leg aids while riding (not the horses mouth)
2. idea #2 - drill team. divide group into 6 riders (or whatever works for you) nominate one a group leader and give him/her a whistle. Have them watch the canadian cowgirls on youtube or visit their site at www.canadiancowgirls.com - then the groups are responsible for coming up with their own mini drill - picking the music is the BEST part! LOL
points given on style, synchronization and costumes etc... it's a great photo op and a wonderful grand finale for the end of camp
This is silly but the kids have fun. We play the Cracker/Carrot Race.
You need one child on the ground with a carrot and a couple of crackers.
The riders start at the other end of the ring, race [slowly]. to the child on the ground.
The horse is feed the carrot and the crackers are eaten by the rider.
When the rider can whistle they may return to the start point.
We have girl guides that come for camp every summer and they always ask if they get to play this game. The horses like it too!