An abdominal workout will assist you in getting strong core muscles. To assist you in your search for abdominal workouts: exercise.about.com/od/abs/ss/abexercises.htm
They're both good but in different ways. Swimming will usually provide more upper body workout, but bike riding will allow you to run errands, maybe commute to school/work while exercising. It's probably also easier to have access to a bike and somewhere to ride than to go to a pool.
horse riding
A good way to not get in a bike accident is to look both ways when crossing the street while riding your bike. to avoid getting into an accident while riding your bike is to be aware of your surroundings.
Increase the resistance on the bike and you'll simulate a good uphill workout.
A stationary bike is good for American football because it helps the players in their workout.
Exercise as a rule is good, and bike riding is a fairly mild exercise form - but you need to consult your doctor.
I had one its slow but good for trail riding
Walking, riding your bike or you could buy a stationary bike. for me riding my bike helps me to stay in shape it could help you to
Some good reasons to start bike riding would be that it is good exercise and would provide a good work out. Another reason is that one can be in the outdoors enjoying the fresh air.
I can tell you the answer for a road bicycle. Every single muscle gets developed with one exception - the abdominal area. I don't think you will ever get as good a workout on a stationary bike, but the principles remain the same.
If you like to use an exercise bike to work out, and don't mind some noise and wind, the Airdyne will give you a great workout. You can buy a good bike at amazon.com