The nearest legal park is Kansas Rocks, in Mapleton, Ks. They are only open on the weekend. They also put on a 101 class every once in awhile. They have some mild trails, which are marked, and they have a map on their site. Here is a link to their site: KS Rocks Park
The next closest is Tuttle Creek, over by Manhattan. It is run by the Corps of Engineers, and the trails there are not rated, as far as I know. There are usually maps of the trails at the entrance, by the toilet, if someone hasn't taken them.
If you are interested, there are jeep and 4x4 clubs in the area. There also a couple of informal groups that get together about once a month. One you can find on this forum, the Northlanders Group, and then there is the JK Owners, that you can find on the JK Owners Forum.