When are you going? The Sapporo Snow Festival is usually around now (first half of Feb), so that should be your top priority - amazing ice sculptures & fun snowy activities.
I have to confess that the 4x I've flown via Sapporo to ski&hike in Hokkaido, I've not stopped over in the city. But I do have some notes from when I was *thinking* about staying a few days (instead rented a car and drove straight off back to the ski areas but to hike in the summer) and I used to live in Japan, so I can offer some general advice of stuff I seek out when I visit to any big city in Japan:
- The Hokkaido Museum's -open air village, sounds a bit like a Colonial Williamsburg or Plimout Plantation equivalent with a replica historic village. I think the museum itself may be interesting as I think it focuses on Ainu (indigenous) culture, which is distinctly different from mainstream Japanese & more in common with Inuit/FirstNations/Native American. (If you've ever watched the animated series Avatar the Last Airbender or its sequel Korra, the "water tribes" were clearly supposed to be something like Ainu).
- The Sapporo beer museum - who would pass that up?!
- The "curb market" which is the retail part of Sapporo's wholesale market (I guess their equivalent to the famed Tsukiji fish market in Tokyo).
- Otaru is a pretty town nearby that's worth a detour.
- Also, I make a point of going to the Sapporo airport at least 4 and sometimes 5 hours before flights because the *domestic* terminal has a fantastic food market - something like 100 stalls of Hokkaido food products (it's famous for all sorts of agricultural stuff & the dairy is amazing. Read
@alison wong's review of Nozawa Onsen and her notes about Japanese vs American milk. The farmers are heavily subsidized so that the cows are all grassfed and freerange in small herds/family farms, which makes for really delicious milk. ) Also 40-50 eating options. The sushi + ramen are uniformly fantastic, and the individual quivering cheesecake is great too - there are always long lines. Also make sure you get some soft serve green tea icecream. There's a little show factory for Royce brand chocolates which is very fun. I once carried a 10lb box of potatoes back to Hong Kong because my sister and I ate a transcendentally good baked potato as a side dish to our sushi, and we had an empty carryon, so why not?
More general advice, not specific to Sappor: Set aside some time just to wander around the department stores in town even if you're not into shopping. The food halls are invariably awesome and it's culturally very interesting to observe the differences. Just buy something for the sake of getting it wrapped - the wrapping they do for even the most modest purchase is a work of art.
I'm a big fan of Tokyu Hands (Tokyu is a corporation, it's not another spelling for Tokyo) which is like if Neiman Marcus or Saks ran a combination of Michaels + Bed Bath & Beyond + the Lee Valley Tools catalogue - it's the most amazing hobby/household/hardware shop in the world....the Japanese take their hobbies *very* seriously.
You'll probably go to an onsen while you're skiing but if it's not real minerally hotspring water and just a glorified oversized hottub at your hotel, make a point of going to whatever is the nearest real onsen town to Sapporo (it can't be more than a hour away, Japan is all active volcanic zones with hot springs everywhere - there are real ones in the middle of Tokyo even.)
