There are tons of small exercises you can do while taking a shower. Drop the Soap might have a slightly odd look to it, but since no one is watching you in the shower you can just go for it.
It’s a basic leg stretching exercise, try to touch the floor without arching your back. Hold the stretch about 8-10 seconds and repeat three times, going deeper in the stretch each time. You will feel the stretch on the back of your legs.