From the bus dropoff at the Mailbox Peak trailhead, it’s a short road walk before hitting the true trail. If you'd like to avoid the roadwalk and you have a car at your disposal, head to the Granite Creek trailhead. The mileage listed here is from the Mailbox Peak dropoff.
If you're using Trailhead Direct, head down the Middle Fork Road for 0.18 mile to a parking area for the Granite Creek Connector. The trail takes off here, paralleling the road for about a mile before switchbacking away. Traverse the hillside, crossing many creeks as you go. The forest here is deep and lush, and burbling water accompanies you as you trek along. 2.8 miles from the trailhead, arrive at a junction. The trail coming from the left is the Granite Creek Trail, which starts at a trailhead, not (yet) serviced by Trailhead Direct. Turn right at the junction.
Now you’re paralleling Granite Creek. Just a tenth of a mile from the junction, a spur trail to the left offers an overlook of the creek, and another tenth of a mile on, a lovely new bridge is a great place to stop for a break. Here, you’re three miles into your hike.
It’s two more miles to your next significant junction. Be aware that this section of trail is prone to flooding and in the shoulder season you may need to ford creek crossings. A little more than a mile from the first junction, look to the right for views of Mailbox Peak. 0.7 mile past those first views, arrive at a second junction. To the right is a trail to Granite Lakes. While it’s a nice day destination, camping is not allowed at Granite Lakes.
If you’re camping, the trail to the left is what you want. It’s 2.1 miles of up-and-down to Thompson Lake, which sits just inside the Alpine Lakes Wilderness in the Mt. Baker-Snoqualmie National Forest. Once you arrive, find a spot and set up camp.