The South Boundary Trail is a Taos classic and a genuinely epic ride, no matter how you do it. The trail traverses the boundary of Carson National Forest from Angel Fire to Taos. Whether you're careening along soft high alpine dirt through the renowned "Heaven on Earth" aspen grove or picking your way down the rocky bits of El Nogal, you WILL be having a blast. While the last few miles are very rocky and technical, an intermediate rider could do this trail, provided they are prepared to walk a bit at the end.
For the full South Boundary experience, arrange a pick-up in Taos and ride the ~25 miles from Angel Fire to the El Nogal Parking Lot right outside of Taos. Another popular route is to start in Valle Escondido, ride up the road to Garcia Park and ride down from there. Many people will also shuttle the descent from Garcia Park down. If shuttling logistics give you an immediate migraine, it's also possible to do an out and back riding up El Nogal, although be prepared -- this is not an easy climb!