"I would probably recommend this trip to a friend."
If you don't mind sharing the trail with lots of horses, this is very nice trip. Sheep lake can be reached from Walupt Lake trailhead, or, if you're lucky and have time, from the Pacific Crest Trail from either White Pass or Mount Adams.
Coming from Walupt Lake, the hike along Nannie Ridge is spectacular. The trail is heavily used by horses but is closed to mountain bikes. We were on foot and found the trail to be a steady, but not difficult, climb. If you're camping, day trips north and south along the PCT from Sheep lake are fantastic, each mile brings a new inspiring vista - no matter which direction you go.
The loop back to Walupt lake lacks the highlights of the Nannie Ridge trail but makes a relaxing, if anticlimatic, trip back to the car. There are some nice camp sites along the PCT section of the loop. The trails on this loop are well marked and easy to follow. Expect snow patches until mid-July - especially during heavy snow years.
Note: This is backcountry wilderness travel. You are responsible for informing yourself of the hazards and taking the necessary precautions.