Nick and I spent a fun day exploring the snowmobile trails around Mount St. Helens. So far it’s the best snowbiking we’ve found near Portland and worth the easy hour+ drive.
Unless conditions are great (they weren’t that day), I’d stick to the groomed snowmobile trails. Drive through Cougar on the south side of the mountain and follow signs to the Climbers Bivouac. Park at the Cougar Sno*Park (bring a Sno*Park pass), grab a map from the sign at the entrance if you didn’t bring one, and have fun. There are 10-15 miles of groomed snowmobile track and lots more if conditions are good.