Idaho City is where they hold an ISDE Qualifier each year, so there is a well developed trail system in the area. The best place to go to find trailheads is to go through Idaho City past the visitor center. About 1 mile out of town, turn right onto the dirt road just before the main road takes a hard left. There are actually two entrances to this dirt road, about 100 yards apart, so take your pick. You will be driving east on the dirt road about 1/2 mile when you drive across on old bridge. Just a little further, the road will split, so turn left. Drive another 200 yards and look to your left. You will see lots of trailheads. Park your rig, and start riding.
As a note, the South Hills just south of Twin Falls, Idaho also offers some GREAT riding. Another great riding spot in the trees is to go south to Murphy, Idaho, and take the old rabbit creek road to Silver City.
Here is my email address rcorwin@rangen.com. Send me an email, and I can help you find lots of trails to ride. Heck, we might even be able to get hooked up for a few rides this winter.
Rick