I haven't been able to keep my 'gym habit' up, despite still belonging to a gym! However, my non-gym habit in the past was the Royal Canadian Air Force Exercises. Look for the book on Amazon, get a cheap used copy (or a new, if they have it) there or at a local library sale. It only takes around 20 minutes to do what ends up being a progressive program of basic exercises. You start with fewer reps, and slowly add on reps until you're doing much more, but still in the same amount of time. No equipment needed. Not much space needed, either. If you don't stick with the program, you don't increase reps. You only increase reps if you're following the calendar, and working out daily. If you 'fall off the wagon,' you continue at the same level until you're back on track, and then you can move ahead.
It works, it feels good, and it makes you want to do more.