This is an interesting mix of basic on-trail hiking and some really fun off-trail ridge travel. Throw in a rennovated lookout and great views to boot. Consider doing it as an overnight just to get the full experience.
Turn onto the Cascade River Road and follow it until reaching Sibley Creek Road. A sign there indicates "Hidden Lake Trailhead". Follow Sibley Creek Road to the trailhead at the end of the road. This road is steep, narrow, and often in bad condition.
The trail starts out in forest, then crosses Sibley Creek and moves into a long, switchbacking traverse of an open slope. This traverse can be hot and unpleasant in the afternoon. It is somewhat buggy as well.
Eventually the trail reaches Sibley Creek again, directly below Sibley Pass.
Continue on the trail until reaching a saddle between the main peak and the side peak with the lookout. This pass overlooks Hidden Lake. There are plenty of potential campsites here.
However, the most interesting place to spend the night would be the restored lookout. It has a double bed, a two-burner Coleman stove, and a ton of atmosphere. The lookout is first-come-first-served, so don't rely on it being available.
There are fine views from the lookout (of course), but the best views are from the peak itself. The scramble up to the peak from the lookout side is very easy.
After checking out the views, continue along the ridge towards Sibley Pass. The scrambling is a bit trickier on this side of the peak, but with some patience to look for the best route it is still pretty easy. Potential campsites are available all along the ridgeline.
For those looking for even more, don't stop at Sibley Pass. The trail continues past the pass and follows a spectacular ridge traverse out towards the Triad.
Drop down to the trail from Sibley Pass. Keep to your left to avoid climbing directly down the creek.
Then hike the trail back to the car.