As far as in the saddle, I try to think about the things I want to improve that day, and come up with some sort of scenario to practice it. For example, with your legs, you can try to imagine holding a $100 with your leg. Of course you wouldn't want to lose that, so try to think about keeping it. However, don't squeeze your legs while doing it. Just a nice, steady contact while imagining that scenario. Hopefully that makes sense, and you can imagine your own or use that one :)
Riding without strirrups is the ultimate lower leg workout. It obligates you to keep your heelsdown down.
There are other excercises like sitting with heels down.
Hope your question was answered.
Get a wooden block about 3-5” in height and stand on it using the balls of your feet only then push you self up and down on the block. Like in ballet. If you do that everyday you should get a strong lower leg.