I'm spending too much money on a trainer right now who has a setup in his garage and driveway. It's money very well spent. Get your heart rate up to 140bpm for an hour, build some muscle, and get all your energy out (and I go with my good friend, so it's like hanging out, too). It's not as good as tennis, but it's made this spring and summer 1000x more enjoyable. I would play tennis a lot more, but my main partner lives 35 minutes away, and his schedule isn't flexible. So we're always trying to squeeze into the narrow window of after the kids are in bed but before sundown...that works ok in June but is pretty shitty in September.