An important part of staying fit for me is having fun. I like to do things that I did when I was a kid like riding my bike or roller skating. I also like to be outside and be in control of the time, so I do these activities work for me because I can do them alone.
Did you play sports when you were younger? If so, joining an adult league is a great way to stay motivated and have fun at the same time. Many of my clients play tennis, basketball and softball well into their sixties. If you are motivated by competition, you may want to consider some of these options.
Social people tend to do better when they work with a group. Every area has running or biking clubs. If you enjoy those activities like I do, check them out. I am organizing a bike club now in my area because convience is everything when working out.
If you are a gym rat and a social person, why not consider joining a small group training classes like TRX Suspension Training or a boot camp class. Good luck and have fun while working out and burning calories.