Eldorado Springs - Cherryvale (18 miles) - Cross US-36 and turn right (through the shopping area). Once you summit Davidson Mesa on Marshall Rd. you are into open country with great views of the Boulder Flatirons. This route takes you up South Boulder Creek to the mouth of Eldorado Canyon. You can turn around at the town's "wall," where the road turns to dirt.
Return as you came or via Cherryvale, which is a very pleasant road, and South Boulder Rd. is ok, too.
Morgul Bismark (14 miles) - This is the classic Morgul-Bismark loop (named for Davis Phinney's pets) - a bit more climbing than the other route, but even better views! (Good views of Eldorado Canyon and the Indian Peaks defining the Continental Divide.) Go over US-36 and continue up McCaslin to Hwy. 128, from there to Hwy. 93. Here you can descend the couple of miles to Marshall Rd., or depending on time of day, just come back the way you came. (The 1½ miles on Hwy. 93 are downhill but without shoulder. Not generally a problem, but you be the judge.)
Give either of these a try and soak up some Colorado mountain scenery!