Pretty much what everyone else is doing—weights (high-intensity type program based on free weights in a power rack) and cardio (intervals). Another thing I've discovered—very unexpected—is badminton: a terrific cardio workout. The game involves much quicker movements than tennis requires, but those movements are large muscle movements (unlike table tennis, where a lot of the movements, in backhand-pushing phases of the volley that you need to go through to set up a forehand hit, are very short strokes). So you can get, in an hour of continuous play, a ferocious interval-type workout.
Something I've done intermittently is toss a ping-pong ball with one hand and try to snatch it out of the air on the way down with the other hand, just as a test of, and challenge to, my eye/hand coordination+reflexes, and similar tasks. Over time, you do get better, just as with balance training in MAs—there's a neurological baseline for those abilities, but they can indeed be improved with practice, I've found (just as all the kids' science museum display on reaction time keep telling you...) I figure there's probably some carryover to other contexts where hand/eye coordination is involved, such as MAs...
