Like the bike idea. Also, so many people think that starting an exercise program means joining a gym and committing to working out a certain number of days per week. This is ridiculous. Take a walk, do simple stretches such as touching your toes, Google search exercises that are gentle and can be done in the privacy of your own home using only your own body or common household items. Check out this website for ideas: BBC - Norfolk Features - Exercises to do at home

Good luck