Actually, walking is one of the best forms of exercise. If you want more out of it, carry some hand weights. Ankle weights are also available.
If you want more exercise outside of walking, try to find something fun to do, like bicycling or swimming or roller blading. Digging in the garden, vacuuming floors, washing windows are all good forms of exercise, too.
You don't need to spend money to get exercise. If you want to, find a friend to share some of these activities with to make them more fun, or get someone to pay you to do their housework.