You don't consider gliding challenging enough?
BB, It's challenging in a different way and extremely satisfying when everything comes together on a good day e.g. fly through a thermal on the crosswind leg after departure, plan a suitable turn and fly into the thermal again at release height, make a quick descent (allowing for engine cooling considerations) without touching the throttle again until short final and be back on the ground in six minutes.
r44heli,
Now in my book, really challenging was the dual flight I had in a Bell 47. I was doing quite well, much better than I expected but I remember having to remind myself several times to relax and not grip the cyclic so tightly. After 20 minutes, my instructor (who was handling the throttle, which really helped) suggested we quit while I was ahead of the aircraft. A good idea as I was getting tired quite quickly. Great fun.