The Owyhee Motorcycle Club racetrack just outside Boise is one of the best I've found in the country. It is challenging enough for expert riders, yet still doable by novice riders too. The main track has excellent dirt and is kept in excellent condition by a crew of dedicated volunteers. The vintage track, built on a hillside, has the perfect mix of sand and dirt that works perfectly for the older bikes. There's also a pee-wee track and a flat track.
It's a great facility that has been in operation since the club purchased it in 1946.