Rollerblading - that's the type of skating that involves four sets of wheels on each boot, but in a straight line- none of this namby pamby '50s rollerskating, its more akin to iceskating on tarmac, but with wheels of course. Rollerblading requires balance and is a great source of exercise, but it certainly isn't close to the monotony of running along on a treadmill.
You can rollerblade in most parks and along the street, with anyone you want. Then once you've mastered the basics of not falling over, and going in a straight line, it's a real breeze and you can get into some more advanced techniques.