The Hilton Helsinki Airport Hotel — Vantaa (located at Lentajankuja 1 in Vantaa, Finland) is about a 3-4 minute walk to the Helsinki airport terminals and 30 minutes by car to the center of the city. This hotel has 5 floors (the 5th floor is the Executive Floor) and a full-service restaurant and bar. I spent 4 nights at this wonderful hotel and would definitely visit again.
Finland is on the Euro so for those of us with weaker currency, it makes fiscal sense to take advantage of the promotions. I highly recommend pre-paying for hotels that have offers which include breakfast (if your travel schedule is not subject to last minute changes). As a general rule, I would not recommend pre-booking a hotel too far in advance (at max, a month or so) just because you never know what can happen (i.e. the hotel goes out of business or starts a major renovation that will affect the quality of your stay). Breakfast tends to be pretty expensive unless you can find a McDonald’s nearby. By booking ahead, you can save up to 20% on the hotel’s current rates. Which you can use towards buying some bear liver pate (which is apparently a Finnish delicacy).