I think riding at times is very demanding. There is a lot to keep track of and little room for error. Riding a challenging road quick requires multiple calculations and decisions per second as does navigating heavy traffic. Because I also pursued sailing and surfing I equate powered flight, at least VFR GA flying, as not as difficult as unpowered flight.I grew up dreaming of being an astronaut. It's a subtle distinction, but I think that the difference between dreaming to fly and dreaming of being an astronaut is that a big part of my dream was not only flying the spaceship, but all the procedural things that come with it. Operating a huge panel of instruments (think Apollo CSM or non-glass STS shuttle cockpit, not the lame SpaceX touchscreens), talking to ground control, doing scientific experiments in orbit, etc.
I have done some gliding as well, mostly when I lived in England for 2 years as a postdoc and flying was too expensive. Gliding is beautiful and gives you that pure feeling of being in the sky with nothing around you, just the whoosh of air. I find powered flying to be fun, too, but more because it involves juggling and managing lots of things like radio, instruments / ILS, maps & autopilot, etc. When it all works well, it's an incredible feeling, but when things are not going well it is very stressful and discouraging.
Between gliding and flying, I think motorcycling is more like gliding...