The local park is one of the best places to keep fit. Use the grassed area to walk, jog or run around. Sit ups, lunges, push ups and burpees can all be done on the grass. Play equipment and benches can turn into step up and chin up machines. A flat wall can be used for wall sits. There are so many different types of exercise that can be done at the park...and best of all its free!
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.
Around Australia an event called Parkrun is spreading. It is a 5km running course held every week (usually a Saturday) and they time your run. The best part is it's FREE! Such a great alternative to a gym as you can still monitor your improvements by your time each week and anyone can do it as people run/walk with prams, dogs and kids.
I find it amusing (not) watching people park their huge black tractors as near as possible to the gym. Why not park further away and walk there?
My exercise regime started20 years ago, walking x country with my horse riding daughter. Now - most cities have wonderful walking paths in parklands and suburbs.