Recumbent bikes have become very popular for the middle aged and older riders. On a recumbent bike you have a very comfortable seat that you can lean back in, quite like sitting in your favorite reclining chair. Your butt is very comfortably cushioned. Your feet are out in front of you, quite like when you're driving your car. Recumbent bikes are the fastest type of bicycle. One reason is that with your legs extended forward and pedaling in front of you, you are able to move much more weight with much less effort than on a conventional bike where your legs are underneath of you. Beginning recumbent riders will tell you it takes a while to develop what are called "recumbent muscles" but once you do you'll never go back to an ordinary bicycle.
Mountain biking is a popular activity that is enjoyed by a wide age range of people. Heavy duty mountain bikes can be quite costly, so it may be best to go to talk to a mountain biking organization about bike rental. You may only decide you need a light duty mountain bike which would also be good for regular riding. Not all mountain biking trails are killers and for experts only - they had to start somewhere too! Mountain biking trails are rated somewhat like ski trails, the easiest is green, an intermediate is blue and difficult trails are pink. Go slowly, mountain biking can be hard on the joints, and you'll need to learn how to handle the bike to minimize chance of injury.
Whichever type of bicycle you choose, just do it! Biking is a great way to achieve an aerobic work out and burn calories at the same time. The best part is that anyone can ride a bike and it's a great family recreation to be enjoyed together.