Hidden Lake Lookout is in the same general neck of the woods as Sahale Arm. I would choose Sahale Arm first.
Ptarmigan Ridge and Hannegan Peak (maybe Goat Mountain, but I'm not exactly sure where that is) are way on the N side of Mt. Baker. If you are going up that way, I would probably choose to hike Skyline Divide or scramble Hadley Peak (accessible from Skyline Divide, but probalby more accessible via Cougar Divide) and just drive up to Artist Point, which is the start of the Ptarmigan Ridge trail. Or if you go up to Hannegan Pass, instead of going left to Hannegan Peak, go right to Ruth Mountain. A glacier, but generally pretty accessible without ropes.
Little Annapurna would be on the long side for a day trip given your distance and elevation desires, but is a nice hike into the upper enchantment basin.
All of the above would be long drives from Mt. St. Helens.
Alta Mountain is a nice hike and not as far a drive. but is not as spectacular as the others. Easily the closest to St. helens and Seattle.
The other suggestion I would make is Gothic Basin with possible scrambles of Del Campo and/or Gothic Peak. A closer drive than all but Alta Mountain.