A personal fav is tack shop. If you haven't heard of it, here's how we play it:
First rider starts with a coin. Rides to a raised platform or something in which to leave the coin, and leaves the coin there. Then they go a little further up to a barrel which has a tack box on it. They pick up the tack box, and move up to a helper, who will put a piece of "tack" (whatever you like, soft is good in case it falls, so pairs of socks, sponges etc) in the tack box. The rider has to return the tack box to the barrel without dropping anything (if they do, they have to get off and put it back in and continue riding). Then they continue to the coin, pick it up, and pass it on the the second rider. And so on.
Not sure how well I explained that.
The balloon race is always fun.
We also play a version of bending which is pairs. You can't let go of the end of a rope you are holding between you and have to complete the bending/point to point, etc. Shorter than a lead rope is good, a longish leadrope in half works. If playing in a four then at each end of the poles one rider switches with another.
Hope that helps, if I think of any more I will come back =]