The Sam Houston National Forest has dirt bike, equestrian, and mountain bike system consists of approximately 80 miles of marked trails. The trails are available for motorcycles, all-terrain vehicles less than 45 inches wide, bicycles, and horseback riding.
How to get there:
For the East trails, take I-45 (about 13 miles north of Conroe and about 10 miles south of Huntsville); exit FM-1375 and go west about 3.4 miles. Turn right on FR-233 Rd (I don't know if the road is marked). There is a sign indicating trails though). Go about 1 mile and curve to the left when the road splits. On the left is a big juvinile facility. Continue for about 1.5 miles. Parking is on the left. They want $5.00 per head to ride. Trails are marked.
For the West trail system, go about 9.7 miles west of I-45 and turn right on FR 215. Go north. Parking is about 3/4 mile on the left side. Trail Map at http://www.southernregion.fs.fed.us/texas/recreation/sam_houston/samhouston_rec_opps.shtml#shmutrail