Stay Fit Without Going to a Gym

With that in mind, here are things you can do that never require a gym or lots of expensive equipment, in order of intensity:
 

1. Stand instead of sit: try a standing desk, or just using your computer/book/phone etc. at a countertop instead of a couch or chair. You can do this even in places like meetings or when you're talking on the phone.
2. Take a walk: a surprisingly large number of the most brilliant and interesting people have had a daily habit of going for a stroll. Walking is the kind of exercise that leaves room for you to think, and doesn't feel like "exercise" in that intense way.
3. Go hiking: just think of hiking as advanced walking, with more awe inspiring views.
4. Do bodyweight exercises: you can do all kinds of strength training without any equipment. Your dad or grandfather called this Calisthenics and you can do most of these pretty much anywhere or any time you like. A good beginner's list to try is the 7 Minute Workout by The New York Times. A controlled study actually showed that even just short daily workouts like this are as good for you as running a few miles.
5. Cycle: riding your bike for pleasure or for your commute is a great way to get aerobic exercise. If you're just riding to get some fresh air and a bit of exercise, you don't need a $3,000 carbon fiber road bike, so I think this still qualifies as much cheaper than a gym.
6. Run/jog: this one is pretty much a no-brainer. Maybe one of the best things about running is that you can do it whenever you want and almost where ever you want. See How do I get started with running?