How about speed work on a snow bike?
Tonight I went out on the snow bike on a firm-ish snow machine trail and did intervals for an hour. Why? I don't know, I was a little bored and had only an hour. On my HR agenda I'm apparently also "supposed" to do intervals now a few times a week. My top speed was a wild and crazy 12 mph... downhill. My slowest speed and highest HR was an uphill 3 mph. The temp are back down to a reasonable -5. It was painful and a few times I was kind of nauseous. It felt weird and oxymoron-ish on a snow bike but having a short intense ride helped me feel better in the end.