Note: Our route description for Mountaineer Peak takes you over another lower-ranked 13 called Mt. Rhoda. Both the trailhead and route descriptions for these two summits will be largely the same.
From the passenger car parking at 10,460 ft., hike along the road as it crosses to the west side of Cunningham Creek and then heads uphill and south. In a little over a half mile, you'll arrive at the 4WD parking and the end of the road which again crosses back over Cunningham Creek. A gate prevents further driving, but the vehicle track continues some distance before becoming more of a trail. Not far up from the upper trailhead parking, the creek passes through a narrow chasm of either granite or metamorphic rock and presents quite a tumultuous sight. At about 11,620 feet, will be the campsite we mention in the trailhead description. It's on the slope of a hillside that runs between the two major forks of Cunningham Creek. On this unlikely looking slope, there are some tall trees and some flat spots to pitch a tent.
The main trail to Highland Mary Lakes turns west here crossing and following the stream, then turns south again to follow the main stream that drains one of the first lakes. A few hundred yards before arriving at that first lake (12,100 ft), cut right (west) up a steeper slope and gain the south flank of a mostly tundra-covered, broad ridge that's east of the Mt. Rhoda summit (UN13,402). Follow the ridge west and contour into a basin on the south side. Stroll on up the lush basin admiring the wildflowers (if you're there at the right time) and at the head of the basin, either hike up the SE flank of Mt. Rhoda or walk to the saddle south of the summit. Since this little summit poses no difficulty other than a very minor cliff band near the summit, it's just as easy to hike over it as around it. If you go to the south saddle, you may wish to contemplate hiking south first to Whitehead Peak (unranked), and then on to UN13,109, before turning around and going to Mountaineer Peak by way of Mt. Rhoda.
From Mt. Rhoda, it's an easy hike across to Mountaineer Peak (UN13,434). There will be some scree and rock mixed with plenty of tundra and the hike over should take under a half hour. The views from any of these summits allows you a panorama of the Grenadier Range to the south, with Arrow and Vestal Peaks standing out prominently.
To descend, you might want to consider an alternative route back. Drop south along the ridge from Mountaineer Peak to the saddle, then descend into the basin to the east. (Spencer Basin) Head ENE across the basin to a ridge and saddle just SW of "Sugarloaf." Cross the ridge, descend on scree and tundra into the basin below and in another half mile, pick up the Highland Mary Lakes trail to finish out your day. When we hiked in Spencer Basin, it was a 4th of July weekend and most of the basin was still filled with snow. We suspect much of the basin, once you're off the peaks, is tundra and grass. If you like to play around in the snow, then this entire area is good for earlier summer season access.