The polo sounds fabulous!
We used to play tag all the time when I was a kid; even out on the trails, if we found a sand pit, or open area the game would be on.
I also like to do games like one barrel pennant, barrels, jumping figure 8, etc...I mainly will do these after a good session, as that is something 'fun' to do afterwards. Basically if you have poles, jumps or barrels or cones laying around, you can devise just about any kind of game you can imagine.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."