It's so hard to narrow it down to one, but if I must, I would say rollerskating. I lived in a hilly area, so it could be a challenging activity.
I've never forgotten the time when I decided to take a neighbours dog for a walk while wearing rollerskates (me, that is, not the dog! ?)
I made it down what was known as 'the steep hill by going at an angle and using the kerb to stop me every few yards. Then as I went up a rough path towards the park, Sandy the dog spotted a rabbit up ahead and set chase. I hung on to the lead and had the fastest, most exhilarating dog-powered ride of my life ?