If you have weak joints a water aerobics class is fun and helps build strength and flexibility.
Get out on the town to a dance club. Get up and dance as much as possible. You do not need a partner. Just get out there and move.
Try skating. It is great exercise and lots of fun.
Tai chi is great for the joints. Join a class or get a tape.
If you can not stand the idea of going to the gym, here are some fun ways to get exercise that will keep you from getting bored, burn calories and increase fitness on every level.
Taking a hike can help to burn calories and get you outdoors in the healthy air. Find some trails or a park and go for a nice long walk. Going over hills is especially good for burning calories.
Ride a bike for short errands or just for fun. Even short bursts will provide aerobic benefit.
Go play some racquetball. You do not even need a partner for this. This is a fantastic calorie burner, and builds aerobic capacity.
Ride a horse. There are many places where a horse can be rented for a one hour trail ride.
Join a softball team. This is a great workout for cardiovascular and musculature health.
Play volleyball. Even if you have one other person it is possible to get a game going. This will help with your vitamin D intake as well.
Just get out there and exercise. There are so many interesting ways to get fit. Whatever activity you enjoy is a good activity to use to burn calories. Most of all have fun! The more you enjoy working out, the more likely you are to stick with it.