Starting from the South Sourdough trailhead, follow the road for the Mountain Research Station up towards Niwot Ridge. Eventually the road ends, but the trail continues on up to the end of the Research Station outposts. You can return the way you came (13 miles, roughly), or take the Niwot Ridge Spur Trail
up to Brainard Lake and the South Sourdough (#835)
Trail back to where you started, for a 20-mile day.