The south facing bank of the Big Smokey River is about 6km x 3km and a drop of 100m. Room for that and a water park and a few other things that you do while on vacation where it is warmer than it is on the flat land at the top of the riverbank. There is also a ridge close by that is in the deep shadows so trails that are frozen would. 5% slope would take you a few miles before you hit the bottom of the run that is suitable for 'toddlers'. The high speed ones have only one exit.
Would a field at 9% be suitable for a 'practice field'? Big leg muscles Yes??
Lots of moss covered swamp for 'drone wars' as well as carving mazes in for the 'bumper-boats'. Come one, come all, come again if you survive the previous visit, with the proper documents of course. There are no 'wounded here'.