I was curious about tent camping in the winter, so I created a new save and started with winter in Granite Falls. I discovered that when it's freezing out, Sims can hike but shouldn't do yoga, and a fast way to warm your blue Sims after ill-advised yoga is woohoo in the tent.
They're perfectly fine sleeping in a tent, and can take a shower in the tarp shower when it's freezing. So long as they're wearing their winter clothing, they're quite happy to sit at the campsite reading. I haven't had a blizzard yet - I'd imagine they'd be confined to the tent. (I'm hoping they won't try to pile up in the little toilet building I built at the site.)
You can plant spinach if the temperature is just above freezing - useful for bait if you want to do some fishing. (Fishing is only possible when it's above freezing, of course.)