Mystic Lake is a proper Mount Rainier backcountry goal, not a quick viewpoint. Reaching it usually means a long approach through forest and alpine country, with the lake sitting below Mineral Mountain and Old Desolate on the park's quieter north side.
The reward is solitude by Rainier standards: water, meadows, and a sense of being well beyond the road-accessed highlights. It is best suited to experienced hikers or backpackers who have checked permits, snow conditions, and the current state of the access trails.