Thornton Peak is the highest point above Thornton Lakes. The SE ridge is mostly class 2, with occasional class 3 moves on a good rock.

Saturday: drive to Marblemount Ranger Station to pick up overnight permits. Drive to Thornton lakes trailhead (215 km). Hike to Thornton lakes camp (8.4 km 2,400 ft (731 m) gain to ridge, 500 ft (152 m) loss to lake).

Sunday: Scramble Thornton peak (6935 ft / 2114 m) and back to camp. Optional: scramble X mountain and Trappers peak. I can find description only for access from the East, but I suspect they can be gained from West too (the col with Thornton)

Monday: pack camp, hike out and drive back.

 Plan B in case there are no available permits for Thornton lakes:

Hike and camp at Stiletto lake (the lake is on the border of North Cascades National Park), scramble Switchblade and Stiletto peaks (class 3-4).

PASSPORT, helmet, overnight camping gear, food for the trip, enough clothes to survive a night in the alpine in a case of emergency.

More info:

https://www.summitpost.org/thornton-peak/957754

https://www.summitpost.org/x-mountain/675346