25 Strong Swings
This is something that can be done while players are waiting for their turn during hitting drills or stations. All they need to do is give 25 good, HARD swings in a row. Not necessary immediately after the other, but one good strong swing, then set up again, and another good hard swing, etc. Each time concentrating on sound hitting form and mechanics as well as swinging has hard and fast as possible.
25 doesn't sound like a lot, but players will probably be getting tired toward the end. Eventually you can work up to 30 or more. You just don't want them doing so much in one set that they are too tired to swing properly and to execute a nice solid, strong, hard, fast swing.