Here is a great trail in the Castlwood State Park! You start off on some nice mellow trail folowing the creek banks over to the Lone Wolf shelter where you are greeted by a 9% grade, half-mile climb! At the top of the climb it gets a little easier, but you're still climbing for a little over a half mile more.
You then come to the castle in the woods. Round the castle and you start back down in to the creek valley below. With some fast trail and tight swithchbacks, you get to test your skills. There are a couple of logs to get over on the downhill and an armored drainage crossng swithchback at the bottom, then back into some more creek bank riding!
You'll come to a section in the trail where you'll have to cross the creek. There is about a 8" or 10" drop into the creek and there is usually around 6 to 12 inches of water 15 or so feet wide to pedal through and you're back into the parking lot.