You might just need to ask her and experiment. Sexual communication is really important. That is part of your responsibility as a lover. So what I would recommend is giving her suggestions. There are really no exercises one can do.
Penises are rather self-maintaining.
You really don't have to exercise your penis. That is one of those things called aphrodisiacs, which are things that get someone turned on. They have been the stuff of myth for centuries. Spanish Fly is a concoction of a crushed beetle and it is a category of drug called Cantharides. Actually, this summer in historical colonial Williamsburg, I went into a 17th century pharmacy shop and a woman handed me a jar of Spanish Fly.