Ride Baby Ride
I rode my bike everywhere when I was a kid, and we had a steep driveway that was both fun and scary to go down. Most kids love riding just about anything with wheels. If you're lucky enough to live on a cul-de-sac or have a wide driveway or other paved area perfect for ride-ons, you're good to go for safe riding (helmets required, of course). You can find toddler-friendly bikes used for very little money, and since they'll outgrow them soon, that's often the way to go. Yes, they make powered cars and trucks, but the point is for your kids to get some exercise and use their bodies for motion.
But let's think big here. Older kids might be into skateboarding, roller skating or BMX bikes. No skate park nearby? Live off a busy road? If you have the means, you could provide your kid with a park experience in your own backyard and be the envy of the neighborhood. Some quick Googling turns up numerous sources of plans for quarter and half pipes -- they're just plywood and two-by-fours. Not a lot of space is necessary, either. With a BMX fanatic, you'll need more space to create a course (a half-acre or more), but then it's just a matter of creating the jumps, which are packed mounds of a clay and sand mixture. No woodwork is required, but it'll take a lot of that mixture.