Your best bet is to stop by Jackson Guard in Niceville and get a recreation pass (usually $10-12 per year). This is required for any person that will be going onto the "reservation".
Theres no rocks to play with but plenty of soft sand and some clay... there are a few shallow creeks to play in as well. If you go north of I-10 there is Blackwater and other trails that can be fun.
There are a few off-road groups around here but I do not know of any specific names or contacts.
check with this group: http://emeraldcoastjeepers.com/
and see if they have any outings or if any members may want to show you around.