Look online for exercises you can do while you're on the in-ground trampoline. You don't have to worry about getting too fancy with your routine, simply jumping is a great work out. You can add in wrist weights or even waist waits if you're looking for ways to amp up the exercise you're getting while you're on the trampoline.
You may find that you look forward to your rebounding work out more than you have ever looked forward to an hour on the treadmill at the gym. If you look forward to a work out you will look forward to and that will be a workout you will likely stick with!