Reimers Ranch in far-western Travis County used to be a private ranch that Milton Reimers opened up for rock climbing, mountain biking, swimming, and fishing. Travis County acquired it in 2006, but hasn't changed too much about the place. Probably the most obvious change is the new ADA trail going in along the cliff rim, which isn't open yet.
Unlike a lot of parks, Reimers Ranch isn't really developed - there are porta-potties, but no running water or real bathrooms. The roads inside the park are dirt, but passenger cars should be able to get to the rock climbing/hiking parking lots and the upper beach parking lot. If the road down to the lower beach parking lot is open, be careful - a tow out of the park costs $300 according to the park ranger we talked to. If you've been to other Travis County Metro Parks like East or Southeast, this is completely different - no soccer fields, no concrete sidewalks, a much more relaxing experience.
The maps of the park that the rangers give out are pretty bad - we're on the lookout for GOOD maps of Reimers Ranch mountain biking, hiking, and rock climbing. The hiking trails are not clearly marked yet, but perhaps with the ADA trail work, more trail markings will come in later this year. The mountain bike trails are well marked and not too confusing - following the beginners loop was no problem, but when we cut from the race loop back to the beginners loop, we ended up on the black diamond trail.
One thing to note about Reimers Ranch is that while it has 18 miles of mountain biking trails, they are NOT shared-use for hikers and runners. The mountain bike trails are for biking only, and are one way. This eliminates conflict between hikers and bikers. Really, the most interesting parts of this park for hiking are along the river on the other side of the park road. Similarly, the hiking trails are for foot-traffic only, not mountain bikes.
Reimers Ranch is one of three good places in the Austin area for rock climbing - Barton Creek Greenbelt and Enchanted Rock being the others. The climbing in the park is down along the river bluffs by Reimers Canyon.