For out of the saddle stuff I'd be inclined to stretch your calf muscles - you can do both together by standing on a step and letting your heels dangle off the edge. Make sure you're dropping your weight into your heels. Also using a step, put one foot on the second step, knee bent and again drop your knee forwards and weight into your foot to stretch,
In the saddle try doing the frog exercise - so without stirrups lifting both knees up into jockey position, then lift both your legs completely away from the saddle and then down towards the floor. Definitely lots of work without stirrups, and when you have your stirrups make sure they're the correct length for you and rather than think 'heels down' think 'toes up'. You won't grip in the same way with your leg thereby reducing the amount of time you spend struggling with stirrups. As for finding your secure lower leg, an old instructor of mine used to have me either with or without stirrups, with legs in jumping position, knees in knee rolls and up out of the saddle in two point. You know if your doing it right as it bl00dy kills!