Some exercises to strengthen legs for riding?

So I've been not riding much over the winter and i'm just getting back into the swing of riding, what are some exercises to strengthen my legs? I know about riding without stirrups and just plain riding will help, but i cant ride every day (only like 4-5 times a week) and i've noticed cant keep my legs still and they are swinging around at the trot and canter (this didn't used to happen before i took my break from riding), I know this is because i need to get back my lower leg strength, any exercises (out of the saddle) that will help?