Once you've completed a challenge, you can warp back to it but not start it. For instance, the challenge 'Thorax Crunch' is on the top of a crane. You can warp to it, and you'll be on the crane. This can help with achievements or having fun.
Need to get on a roof? Go into object dropper and drop a slide (in ramps). Place it, then run to the top. Set a session marker. Run back down, drop another slide, place it upside-down on the first one. Drop one right-side up on top of that one. Go to your sesh and you should be on top of the top slide. Repeat as many times as you wish.
If you need to hit a ledge/rail that's too high, or whatever, you can get off you board and run towards the ledge. When close, jump and get back on your board. You'll land in a grind, and be closer to owning that challenge.