How much do you want to spend? Given that your stay is going to overlap two "hotel" days, it might be best to call hotels near SeaTac and see what they can offer. 3* hotels include Doubletree, Hilton, Marriott, Coast, Holiday Inn. There is also a Holiday Inn Express, a Hampton, and other 2* hotels near the airport. All have airport transportation. I've been getting the 3* hotels regularly for $40 - $50/night on Priceline so another possibility might be to bid for 2 nights on Priceline and then call the hotel and tell them you'll be checking in really late (i.e., in the morning) for your first night.