The home gym is a classic in spare room usage for a reason. A home gym can be anything from free weights and yoga mats to a full-scale setup of treadmills, weight benches and other exercise equipment.
Add a wall-mounted TV for workout videos, or a sound system for your favorite tunes to keep your blood pumping.
Whatever you do, make sure the floors are sturdy and well-protected from whatever you might be doing. A consultation with a fitness trainer, even online, can help you design the ideal workout room.