Lake Serene was the destination of my second hike in the Alpine Lake Wilderness in Washington (the first being Snow Lake which was magnficient since there was still a lot of snow left). It's a round-trip hike of about five hours, covering thirteen miles/eight kilometres, gaining over 600 metres/2000 feet, with the high point at 772 metres/2550 feet.
For those who are process-oriented, the path to Lake Serene is beautiful, starting in the wilderness area with mountainous growth and climbing to spectacular views of Bridal Veil Falls (a short side trip I highly recommend taking). From the Bridal Veil Falls junction, the trail climbs steadily upward, presenting awesome views of the Monte Criso peaks as well the walls of Mount Index.
The lake itself, surrounded by Mount Index, is extremely still and reflects very well and lends itself to deep contemplation.
The climb is strenous, particularly if you do it in a hurry, but it's worth it both for the views and the workout.