Take your snake swimming! If you have the space, a molded plastic kiddie pool or a vinyl-lined pool will work just fine. Fill the pool with enough warm water so that your snake is able to keep his head above water. Put him in the water for supervised exercise sessions, allowing him to swim for 15 or 20 minutes several times a week.
It's best not to swim your snake in the family bathtub but, if you must, make sure you thoroughly disinfect it prior to allowing human family members to bathe (one tablespoon of bleach per gallon of water works well).
For more information, please read Reptile Exercise.