Serene lake is a backpacking trip that I've done a few times. It features wonderful Oregon moss-y forests, a variety mushrooms, huckleberries, and some wonderful lakes. If you're lucky, you can also catch some mountain views from ontop of the hike's one ridgeline.
The hike is lollipop shaped from the entrance and clocks in at around 14 miles. It starts at Shell Rock lake. There are often car campers here, given it's proximity to the parking lot (about a mile or less). From there, you take the trail onward to a fork in the road. The first year, we went right, which is the direction I'd suggest. It takes you to Frazier Turnaround, which is accessible by car some parts of the year. Commenters saying that it's blocked by boulders and such has been enough to add the extra 2-3 mile hike for the Shell Rock start.
After Frazier turnaround, it's a mile to the second set of lakes: upper, middle, and lower rock lakes. These make perfectly fine campgrounds. The middle lake is bigger than the lower lake. When I was there.
Continuing on, there is Serene lake, a glacial lake that's clear and a perfect blue. It's cold, but swimmable on a warm day. There's a rocky outcropping in the water that can be a good destination to swim to.
From there, you ascend a bunch of switchbacks 1.5 miles or so to get to the ridgeline. This has a good view of Serene Lake and you can catch mountain views when the skies are clear.
A note: the ridgeline gets cloud-y because of the elevation. You should absolutely put on your rain fly and such. The moisture condenses on the trees and drips on you. It kept us in our tents far longer than was reasonable because we thought it was spitting rain. Walking out to the bare patch of the ridgeline made it clear there was no rain.
After this, it's almost all down hill. You'll descend into Cache Meadow, which has tall grasses, wildflowers, lots of buzzing insects. Some times of year there is standing water with lilys, but others it just grass.
A short climb out of Cache Meadow will take you to a section of trail absolutely filled to the brim with blue huckleberries, which one of my camping companions called "infinity berries". It's short work from there back into the split-off just before Frazier Turnaround, down to Shell Rock, and back to civilization.
If I had to do it again, I'd probably aim to do the following loop:
Shell Rock -> Frazier Turnaround -> Middle Rock lake -> Serene lake -> Lunch around 2-3pm and maybe go for a dip. From there head up to the top of the ridgeline to setup camp. Then it's a reasonable day the next day to do the rest of it and head home.
This was a cold trip in the first week of September. I didn't have the gloves or long underwear I should have. We wanted to get a backpacking trip in before the weather turned bad, so we just went with what we knew. We reversed the direction to change up the pace.
This meant the first day we went through infinity berries, and camped on the ridge line. Most of my group had bags full of berries, adding to their weight. The next day, we had a short day and camped at Middle Rock lake. It was so short, that we were all cold from sitting around for so long.
I learned that finding trail conditions and temperatures is difficult. You have to find things approximately in the same place, but preference for finding things at the same elevation.
From the gear side, I need to buy some 2-3mm line so I can splay out my tension-based tent a bit more which would have helped in the rainy conditions. I also need gloves, to bring long underwear, and to pack a few extra snacks. I think my caloric goal is closer to 3,000 calories.
This was one of our first backpacking trips. We camped at Middle Rock lake the first night, swam in Serene lake the next afternoon and spent a second night on the ridge line.