The only one i can think of right now is the "Heubach" Park
But its pretty far from Jena because Jena is in the North of Thüringen and there are barely any mountains there.
Check out freeskiers.net
You can probably find someone from Thüringen there, who can tell you where to go and maybe you can team up with them!
Good Luck and Ride on!
This is for Shane,CR,Jamie and Sarah
Ski in Peace