If you have a resistance band (or any sort of strong elastic), make a small loop with it. Tie one end around the leg of a chair, and put your foot into the other end. Then just sit on the chair and pull your leg inwards until it's level with your body and release, repeating until tired then swapping leg. It mimics putting on your leg on a horse but with enough resistance to build strength quickly
Squats and line jumps are helpful but swimming builds long my which is what gets used in riding whereas most other things build bulk muscle which doesn't help much however if you are not going to do swim correctly it's better to stick with other things.