Smupiter altitude challenge
Seems simple enough, and based on my experiences escaping from SOI should be neigh impossible.
It would also push rocket builders to the limit; it takes careful design to prevent the rocket from crushing itself prior to takeoff, careful balancing of thrust to prevent it accelerating too fast in atmos, thruster positioning to prevent it ripping itself apart... I can land anything, anywhere and if its an inner smanet, return my budding little commies back home again too, but i can't push past 3k above the surface.
And I know of only one redditor who has achieved 10k, and he's been at it for aaages... this is a solid, difficult challenge.