There is an interesting and highly effective variation to the squats and push-ups you mention: do them in isometric mode, that is, do a wall squat, hold the squat position until your legs 'burn', and you'll have experience that reassuring muscle soreness the next day that tells you that you have given your leg muscles the right stimulus.

You can do the wall squat while brushing teeth, for example (you do brush for 2 minutes, hopefully).