Chambers Lakes Basin Trail is a 13.5 mile lightly trafficked out and back trail located near Sisters, Oregon that features a lake and is rated as difficult. The trail is primarily used for hiking, walking, nature trips, and birding and is accessible year-round.
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.