Take a celebrated route like the Standard East Face
on the Third Flatiron; subtract the crowds and the eyebolts, keep the quality, and add 350 feet of climbing and a little bushwhacking. The result is the East Face South Side route--not only the longest route around Boulder (1300 feet), but one of the very best easy climbs in the area.
All routes on the Third Flatiron, including this one, are subject to seasonal closures. Access is usually forbidden from February 1st to July 31st. Check here and here for up-to-date information.
It is fair to assume that the approach is responsible for the relatively low traffic on this route. Instead of the well-marked trail to the East Bench, one has to follow a steep, very faint path that eventually peters out. It is only a few minutes' bushwhack, though. Start from the Chautauqua parking lot, reach Bluebell Shelter, and take the Royal Arch Trail. Continue on that trail past the fork whose right branch is marked as access to the Second and Third Flatirons. About 50 feet before the crossing of Bluebell Canyon, a very faint trail climbs up toward the Third. Follow this faint trail, which skirts a talus field, contours the Third Flatironette, goes though some boulders, and finally deposits you at the base of the Third Flatiron.
From the base of the Third Flatiron, climb an easy ramp that angles right to a shallow bowl. Since the base of the rock is infested by ferns, it is convenient to rope up at the bowl; from there, the summit can be reached in six pitches with a 60 or 70 m rope.
The first pitch takes you under an overhang that crosses the lower part of the east face. The overhang can be circumvented on the left on the very edge of the face (5.5) or attacked directly via one of two cracks (5.7). There is no consensus on which crack offers the easier passage. Expect to use every meter of your 60 m rope on this pitch. If you start from the base of the Third, two pitches are necessary to reach the overhang even with a 70 m rope. We went by an old unreliable bolt, but we did not clip it.
On the second pitch
Above the overhang, one climbs for three long pitches on flakes and textured slabs (5.easy). Multiple lines are possible: Ours stayed close to the edge of the face and went by the top anchor (two bolts and chain) of Shoyu State (a 5.11a route on the south face). Another party that had started before us kept further to the right. At the end of the fourth pitch you will find yourself to the left and above the giant C painted on the east face. You may spy the last eyebolt of the Standard Route near the top right corner of that C.
The fifth pitch presents one with a choice. One can keep to the left and climb the secondary summit known as Dog's Head and from there continue for 250 feet to the summit. Alternatively, one can angle right towards the top of the Gash. A last pitch crosses the Gash at the height of the South Bowl, and merges with the direct finish to the Standard Route (5.4).
The descent from the Third Flatiron is not trivial, but is well "documented:" Instructions accompany the bolts for the three rappels. See the Standard East Face page
for a detailed description and a diagram
Rappelling from Friday's Folly Ledge
In summary, two short, southbound rappels take you from the summit to the South Bowl, and from there to Friday's Folly Ledge. From that ledge, rappel west from the western of two bolts. Free soloists without a rope usually downclimb either the Standard Route or the Southwest Chimney. There is no walk-off from the summit of the Third. The third rappel deposits you at the saddle known as West Bench. Hike north along the trail that descends to the Royal Arch Trail.
As is often the case on the east faces of the Flatirons, one places only a few pieces on each pitch. Large cams are often handy, but they are not indispensable on this route. Opportunities to place pro are more abundant near the left edge of the face. Numerous chickenheads are waiting for you to sling them.
Good sources for the routes on the Third Flatiron and its neighbors are the guidebooks by Gerry Roach and Richard Rossiter. See also Mountain Project's page