Hmm, try a little of everything. Go to a little cattle sort and see how your horse takes to cows, set up little jumps and jump over them. The best way though is to ask your trainer what your horse is built to do.. and see if they like that.
It's more about enjoying it more than anything. My horse and I are doing "Dressage" but I plan on playing in the Western world with him. Maybe your horse would like to try some endurance. Won't know until you give it a go.
"Strength is the ability to use a muscle without tension"