This is the trail from the Old Penitentiary up to Table Rock. I like to take The table rock triail but you can take quary trail around and down from the top to make it a loop. I like the loop best.
From Downtown Boise take Myrtle St. to Broadway to East Warm Springs Ave. to N. Old Penitentiary Rd. Left turn to the parking area.
http://www.singletracks.com/item.php?c=1&i=2261