Get a membership to your local recreation center. When the weather is foul, go to the recreation center for swimming, basketball, racquetball, indoor track, or any of its other offerings. Use the recreation center as an incentive for getting other work done: "When your math is finished, we'll go swimming for an hour."
Check out exercise DVDs from your local library. Getting workout DVDs from your library or from a streaming service such as Netflix allows you to have a variety of workouts that you can do inside. Buying them is less effective because kids will easily get bored from doing the same workout over and over again.
Ride bicycles to run errands. Instead of loading the kids up in the car for a trip to the grocery store, grab the bicycle helmets and turn the errand into a P.E. class. If you have toddlers, invest in a bicycle trailer so they can enjoy the trips as well.
Explore the local flora and fauna. Combine science with P.E. class by exploring local trails and open spaces. Have your kids look for wildflowers, tracks, bones, rocks, and fossils as they hike in the areas around your home.
Sign your kids up for sports teams. You can count time spent in team sports practices as P.E. hours for homeschool. Get a catalog from the YMCA or your local recreation center to see what sports are offered in your area.