Observation Rock is an apt name for this remnant vent on the north side of Mt. Rainier, about 4 miles NNW of Rainier's summit crater. With 244 feet of prominence, it is just significant enough to be a notable feature in the area. There are other examples of old vents in the area--most notably Echo Rock--but Observation Rock is the largest. It is the most prominent point on the lower half of Ptarmigan Ridge. Understandably, Ob Rock's summit provides unmatched views of the north faces and glaciers of Rainier. You could sit on the summit for hours examining details of Rainier. You can observe four famous ridges, at least five glaciers, three walls, and two ice cliffs...
Ridges: Curtis Ridge, Liberty Ridge, Ptarmigan Ridge, and Sunset Ridge.
Glaciers: Carbon Glacier, Russell Glacier, North Mowich Glacier, Edmunds Glacier, and Liberty Cap Glacier
Walls: Willis Wall , Liberty Wall, and the Mowich Face
Ice Cliffs: Willis Wall Ice Cliff and Libert Cap Ice Cliff (both part of the Liberty Cap Glacier)
Willis Wall is the largest, highest, and perhaps most imposing face in the state, if not the contiguous U.S..
Observation Rock itself is a crumbly, craggy formation. Its origins are similar to another volcanic vent: Little Tahoma. As such, scrambling on it away from the beaten down path can be exceedingly dangerous. I wouldn't venture too close to an edge to get that "better" view. Even with its friable nature, Ob Rock is a popular objective. It even boasts a beginner ice climb on a north-facing ice sheet. The peak presides over Spray Park and is an easy to moderate scramble from there. The peak can be climbed in a day from the Mowich Lake Trailhead. Ice axes and crampons are needed later in the season on some of the icefields around its base (this is the Flett Glacier). Some crevasses exist but these are mostly avoidable (unless it's a whiteout in which case there's not much reason to go up there anyway).
The shortest approach is from the Mowich Lake Trailhead, but you could also get there via the Carbon River Entrance or the Wonderland Trail. Here is a handy map of the park showing the area of concern.
To get to Mowich Lake, first hope that it is late enough in the season to even be able to drive all the way there. Mowich Lake is at 4,930 ft, so it and the approach road can be snowed-in until July. If the road is open, then you first want to find a way to the little town of Carbonado (el. 1,200 ft) northwest of the park. From Carbonado, drive Hwy 165 south. In three miles the road will cross over the Carbon River on a high, narrow bridge. In another 0.6 miles you will come to a fork. The left fork goes to the Carbon River Entrance to the park. The right fork (Mowich Lake Road) goes to the Mowich Lake Entrance. Take the right fork. Around this road are private timber lands, so you don't have to worry about getting lost. The road only goes to one place: the lake. Not too far up from the fork you will get your first view of Rainier looming over Poch Peak in the foreground. Observation Rock is briefly visible up the Carbon River valley. In 11 miles you will come to the park entrance. A little further on is the parking area for the Paul Peak Trail. Six miles from the park entrance is the parking lot at Mowich Lake. If you arrive here late in the day on a summer weekend there may well be 100 cars there.
You want to take the trail to Spray Park. From the parking lot, a short trail connection southward on the east side of Crater Creek will junction with and thereafter become the Wonderland Trail. Continue on the uphill (left) fork of the trail. The trail bears southward for a mile until arriving at Eagle Cliff, whereupon it turns eastward and switchbacks steeply up for another two miles or so to where it finally opens up at Spray Park (c. 5,800 ft). This is a lovely area when the flowers are in bloom amidst remnant snow patches in the hollows. It is about here that you will see Observation Rock to the right. Now there is a trail toward Observation Rock but I have not been on it so I'm not sure where exactly it is. A low ridge can be seen extending north from Observation Rock. The trail may be on the other side of that ridge. The way I went was directly through Spray Park (I was doing the ice climb, which more or less requires one to take that route to save time) but the Park Service frowns upon this. The flora in Spray Park is easily damaged and who wants a bunch of trails winding through it anyway. Maintenance personnel have been trying to revert these spur trails back to nature by laying down hay in a wire mesh to spawn the growth of vegetation.
It is about 3.5 miles to Spray Park from the trailhead and maybe another 3 miles to the summit. Once you pick up the trail to Ob Rock, follow it (ask others in the area if they've seen it or inquire with a ranger; there is a ranger station at Mowich Lake but it's seasonal). Eventually, the trail will "end" at the low-angle easternmost icefield of the Flett Glacier. This icefield is the one that lies between Echo Rock and Observation Rock. Now the north side of Ob Rock is very precipitous and loose. The trail to the top doesn't go up this way. Instead, the idea is to ascend the icefield to the 7640+ Echo-Observation Saddle. From the saddle, turn right (SW) and make a long arc around to and up the south slope to the summit. The rock is loose pumice. Some minor scrambling (class 3 at most). Reverse the route home.
The peak and trail are within Mt. Rainier National Park. Whatever the regulations are for the park, you should follow them. Don't cut switchbacks, don't camp in undesignated areas, don't trample vegetation, don't cuss out rangers, don't squeeze the Charmin. You know, normal stuff. Oh yes, you'll need a pass to get into the park, but that goes without saying.
When To Climb
The major hurdle here is access to trailheads. The Mowich Lake Trailhead is at 4,930 ft, as such it can remain snowed in until as late as July. The six-mile approach to the peak is also a daunting undertaking if there's snow the whole way. Access in early season via the Carbon River Trail is also possible (the trailhead is only at 2,300 ft), but the approach to the peak is three miles longer and requires 2,600 more feet of gain. Consequently, the best time to climb this peak is from July to the first autumn snows. The ice climb isn't an ice climb until fall. Before that it is merely a steep snowslope.
Consult with rangers.
Pictures of Echo Rock
Observations from Observation Rock
See also this photo.