you could try: vaulting
bareback riding
doing bounces(multiple little jumps in a row)
obstacle courses
teach your horses how to bow or hug
make up quadrille patterns
try to name/label all of the horse body parts w/ tape on the horse
mini show
groom the horses like your getting ready for a show
act like jockeys (but be nice to your horses :) )
have a photo shoot w/ you and the horses
scavenger hunt
jumping bareback (of course with a helmet)
riding with your eyes closed at a walk and have your friend guide you
with their voice through an obstacle course telling you to turn or something
Thats all i can think of now but ill add more later
Hope that helps :)