Mount Solitude can be reached from Pitkin , Bighorn and Boulder Creek Valley.
Boulder Creek: accessible via Rock Creek Trailhead - whatever you read about the Rock Creek Road being a 4 x 4 road - this is wrong. The only bad part is a rocky stretch of 20 ft right before the parking lot - tons of options to pull out before that ( i saw RVs being pulled up that road):
Access via Gore Creek & Boulder Creek Trail- Boulder Creek Trail is not maintained anymore past Boulder Lake by NFS - trail is now dissapearing under tons of falling trees - so this will be a long and tedious bushwhack. No save descent down from Mount Solitude back to Boulder Creek Valley - scramble North along the ridge via Vista Peak to Usable Pass and bushwhack back down the valley back to Boulder Lake.
Bighorn Creek: no overnight parking is allowed anymore at trailhead - overflow parking at the interstate exit does not allow overnight parking either.
Free overnight parking at Village Center Parking Structure - take East Vail Bus close to trailhead
Access to summit via Climbers's Point or climb over to Boulder Creek Valley via Bergschrund at the most upper part of Bighorn Valley
Pitkin Creek: parking is also limited to 3 hrs
Access to the summit via its Western grassy slopes (basically a walk up) or via Usable Pass and trailless to Mount Solitude's East Face.
You will hike almost entirely within the Eagle's Nest Wilderness - please abide to all wilderness regulations to keep this place wild and pristine