I would definitely not stay at the airport hotel for two nights.

We arrive at about 9.00 pm so it is easier to stay there after our long flight. Staying at the airport hotel ONLY makes sense for people coming in late afternoon or the evening of the 18th particularly if it is after the long flight.

You could use this company to transfer just the two of you to Otaru and do a little tour of Sapporo on the way. The prices are reasonable enough you do not need to do a "group tour" just do your own thing.