This is the most common route up the mountain during the Winter and early Spring. Start at the Marble Mountain Sno-Park (2680') and follow the trail through the woods. Break out above treeline near 4600' and head up the fall line (generally aiming North), and connect with the Monitor Ridge route (might not be obvious) at about 7000', with Swift Glacier to your right. A little over 1000' of uphill from here gets you to the crater rim. Turn left (West) and traverse the rim for about 0.3 miles to make the true high-point on the mountain.
Return along same route - lots of easy glissading to be had in the Winter/Spring. Also well suited to a ski descent.
Trailhead Coordinates: 46.131,-122.171 (Marble Mountain Sno Park). From I5, take WA 503E and drive through Cougar, then turn left onto Forest Service Rd 83/FR8300. About a mile after the intersection with FR8100, look for a large parking area on the left side.
Ice axe and crampons are generally more than sufficient for this route. Stay at least 15' from the rim to avoid standing on potentially unstable cornices.