The Renfrew Glacier is South of the Collier Glacier on the Northwest side of the Middle Sister. It is generally crevasse free and is good for late season climbing when the Hayden Glacier gets chopped up pretty bad. This climb is about 18 miles round-trip with a little under 5500 feet of elevation and you can go car-to-car in 12 hours or so. However, you might want to consider getting a permit to camp in the Obsidian Limited Access Area and break this up into two days.
The first several miles of this route are exactly the same as the Collier Glacier route. Start at the Obsidian Trailhead on Highway 242 and hike several miles through forest and lava field. At the end of the large lava field, you come to the first stream crossing and a junction 100 yards past the stream. I believe this area is called Sunshine Camp. Take the trail to the left and follow it East for less than a mile to the Pacific Crest Trail.
This is where the Renfrew Glacier route differs from the Collier Glacier route. When you reach the PCT, turn right (south) and follow the PCT for just short of a mile until you reach a spring (called Sister Spring). If you hit the falls, you've gone just a bit too far. This spring is pretty much your last shot at water (except melting snow) so fill up on water if you need it.
From the Sister Spring, follow a climbers trail East up to the Renfrew Glacier and eventually the saddle between North and Middle Sisters. The trail starts up a gully South of the large cliff band. The trail goes up the gully and onto a flat, open area below the vast talus fields of the West ridge. From here you should be able to see the Renfew Glacier about 1000 feet uphill and due East. There are several paths that pick through the boulders and talus so just pick one and start hiking up. You want to angle your approach so that you gain the glacier at it's Westernmost (and lowest) point.
Most people will want to crampon up when they reach the glacier. Falling and crevasse danger is quite low on the Renfrew, but crampons tend to make the hiking much easier. The Renfrew takes you about 1500 feet up to the North Ridge where it peters out and you begin hiking again. There is another 1000 or so feet of miserable scree-slogging to the summit, but the view is well worth it. Retrace your steps for the descent and you'll find great scree-skiing and a bit of glissading. Then a long march back to the trailhead.
Crampons make climbing on the glacier a thousand times easier. You may want an ice axe for glissading, but most people just climb with trekking poles. For the most part, the Renfrew Glacier is fairly gently sloped and there is little danger of a fall turning into an uncontrolled slide.
If you feel like lugging a helmet up to the Glacier, you may want to put it on for the scree slog to the top. The rocks are very loose and frequently start sliding down the hill a bit. However, the slope is not terribly steep and most sliding rocks probably won't bounce high enough to hit your helmet.
Leave your ropes, harnesses, and pickets at home for this one.
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