Enter the park using the Nisqually gate. Travel past reflection lake to the Bench Lake trail head. The hike in is about a mile and a half I believe, and it isn't steep.
This is one of my favorite locations to photograph Mt. Rainier. It's just far enough off the road to feel secluded from heavy population, but not far enough to be too challenging. In the fall, with a little patience, the clouds and fog will sometimes clear enough to get a glimpse of the mountain. If you're for lucky, the lake will be calm and provide a great reflection.
Nikon D7100, Tamron 10-24mm, Tripod, Graduated neutral density glass, shot at 10mm, F-8, 1/40, ISO100