Middle Sister Trail is a 16.4 mile lightly trafficked out and back trail located near Sisters, OR that features a lake and is rated as difficult. The trail is primarily used for hiking and rock climbing and is accessible year-round.
Middle Sister is a 16.4 mile out and back trail located near Sisters, Oregon and is rated as moderate. The trail is primarily used for rock climbing and is accessible from July until October.
This is a mountain summit, and it's definitely "strenuous" ◊◊ from the marked Pole Creek trailhead (or from any other).
Most people backpack in and spend the night at Camp Creek before summiting via the south ridge, but it's do-able in one day via that route or by going straight up the Hayden glacier (north ridge approach). The latter requires roping-up and do some glacier travel (for a small glacier, the crevasses are substantial). If trying to make it a one-day ascent, well, get ready for a long approach and not to take many breaks.
There's also a long 15+ mile western approach and route from the Frog Camp / Obsidian Falls trailhead that crosses Renfrew Glacier. Eventually, the Frog Camp route also uses the north ridge approach to summit.
If you're looking for a walk-up, South Ridge is the way to go, but it's more akin to St. Helens-esque boulder and rock-crawling than the mellow ease of something of a South Sister slog. Middle Sister is great, but don't expect trails everywhere: you'll be climbing a mountain, not hiking around a tourist park.
Bring some thick gloves and every ounce of energy you have if you're ascending via the south ridge: the rocks are sharp and steep.
Regardless, it's a wonderful mountain and blissfully devoid of tourists. You might meet some new friends along the trail, but the tour-busses and trail runners will be miles away--and that's a good thing.