It obviously depends on what your goal is (i.e. weight loss, muscle building, or just getting general exercise), but I think it's always a great idea to get a hobby that is also good for your fitness.
Bike riding is useful, fun, and good for your health. You could also add in more walking to your day to day life, or start hiking. I find it's much more fun to explore the world around you than staring at the blank wall of a gym.