Horse Ridge is about 15 miles east of Bend on BLM land and offers some excellent riding. Probably the main reason it is not more popular is the distance from town. The riding is through an open juniper forest on the north side of an east-west ridge. The views are excellent for much of the ride including Bend and the Cascades. This is a common off season destination mostly because it is out in the desert and has little snow but Palm Springs it's not. It is usually windy on top of the ridge so wind protection is recommended in the off season. Although most of the trail is relatively smooth dirt, there is a fair amount of good technical riding in rocky sections. The lower flanks of the ridge can get sandy especially in aptly named Sand Canyon. Unfortunately none of the trails or intersections are signed. if you use the pdf map listed on this page you should be able to navigate just fine without them hopefully.
There are a couple of options on where to start that will affect the difficulty and length of the ride. If you start at the east end of the ridge there is relatively little elevation gain or loose sand, but experienced riders likely find the ride too short. If you start at the bottom of the ridge, there will be a climb to the ridge either switch-backing up the trail or taking the old highway east to the east end of the ridge. The initial climb from the lower parking area is about 1000 feet. The upper portion of this trail is rocky and technical. If you have riders of different abilities and conditioning, it is a short drive to drop someone off at the east end of the ridge.Speaking of rocky and technical there are a lot of angular embedded rocks that couldn't be better designed for pinch flats. Full suspension bikes help a lot with that but you still might want to make sure your tires are fully inflated and maybe run a bit more pressure than usual especially if you have a hard tail. If you are doing the Sand Canyon descent you could stop and let air out about halfway down for the sand traps.
A good ascent route other than the old unused highway is to go up 1B on the map. The climbing is fairly gradual and the rocks are entertaining. If you get to the pavement of the old highway, you missed the turn for it.
Climbing up Sand Canyon is not recommended due to the sand, even after a rain. Descending is not all that great either really although some folks like it. The trail runs straight down with few turns and consequently is getting washed out a bit which adds to the sand at the bottom. You end up ridding the brakes a lot and wasting most of the effort you put into climbing the ridge. This trail is probably an old horse trail or motorized dirt bike trail probably not made by professional trail builders with the BLM. You may enjoy it if you have real fat tires and are happy letting them run at 50 mph.
Even though being out east in the desert it is going to have less snow than say Phil's trail it can still get muddy there in the "off season". That happens when the ground on top thaws but down a few inches it is still frozen. Normally this is very well drained soil but if it is frozen below the moisture has nowhere to go so you may want to stay off the trial in these conditions. There are some nice dirt roads around to ride like the ones to the west of the ridge.
At the end of the trail on top of the ridge there is a good snack or lunch spot with great views southwest to Paulina Peak and southeast to Pine Mountain. It tends to be breezy which is nice in the summer but maybe not so great if it is only fifty degrees out. It is possible to ride off the south side of the ridge, and make a loop back to the other side of the ridge on dirt roads but, the ride down is extremely loose dirt and there are no single track trails at the bottom. There are some lonesome connecting dirt roads to loop back around to the north that you can see from the ridge. You would, however, be missing some excellent single track descents by not doubling back on the ridge trail. To the south there are some single track and ATV trails however they are in some areas with sensitive species and private land. The land owner who also has grazing allotments in the area has experienced problems with gates not being closed and so on. The cattle grazing out there are not sensitive species do a lot more environmental damage than any ATVers or mountain bikers are doing however, cattle are very well organized and have considerable clout in Washington DC especially since their ancestors have been over grazing the desert west for many proud generations. Never the less the BLM does not want to encourage riding in the area unless the routes have gone through their extensive review process. On a more positive note there has been very little grazing allowed in quite awhile on the main ridge and the bunch grass is tall and beautiful. The BLM now has a map available of this area online. Unfortunately unless you have a large format printer it will not scale well to 8.5 x 11 (this appears to be a D size map at 22" x 34"). Note how there are no trails running through the Natural Area even though they are there. At some point this issue will have to be resolved. If you really want to be proper supposedly it is ok by the BLM to walk your bike through the research area. Some of the non-motorized trails showing on this map to the west of the ridge are likely either used by horseback riders or not used much at all and would consequently be very soft either way, unless they get a lot of use by bikers to pack them out.
Getting to the New Official trailhead:____ In Google Maps copy and paste "Horse Ridge Frontage Rd @43.94269, -121.04309" as a destination. The BLM is finally providing some parking and a kiosk as of February 2010. The new trail head is about a half a mile down the road from the old one. The BLM was getting too many complaints from truck drivers about having to look at mountain biker apparatus when they were changing clothes so close to the highway.
Getting to the east end trailhead:____ In Google Maps copy and paste "County Rd 2015/Fort Rock Rd @43.905600, -120.997000" as a destination. From here double back on the old highway about a hundred yards to the top of the first rise. The old highway at the top is not in Google maps but the actual gps coordinate is 43.90931, 121.00277 Again, this trail access point is not signed and this one in particular is hard to spot. There are currently are some rock carnes, but they may or may not be there when you show up.