During the off season a good all round conditioning program would be good. Try some body weight exercises, like pushups, pullups, parallel bar dips, reverse pushups, etc. Most important for baseball is good flexibility and trunk strength, for hitting and throwing. The one muscle group most important for hitting is forearms. The wrist roller is the best piece of equipment for that purpose. You can buy one or make one yourself from a 1" diameter piece of dowell with a screweye to attach the rope and a snap hook on the end of the rope to attach weights. A flat surface to rest the forearms on to isolate them is good, but not necessary. A set of bleachers or a picknick table found on most playgrounds can suffice for that. You can kneel on the bleacher step and rest your forearms on the seat, while allowing the weights to hang below the bleachers. One set up and down with arms both pronated and supinated with about 3-4 feet of rope daily is all that is needed.