Hokkaido is a natural wonder of Japan with amazing snow during the winter and some of the best camping and hiking in the summer. However, coming from Miyagi, Hokkaido is a little pricey to get to. Unless you take the Taiheiyo Ferry. Taiheiyo Ferry services the ports of Nagoya, Sendai and Tomakomai. For prices as low as ¥4000 one way you can get a ride from Sendai to Hokkaido in about 14 hours. The trip is done over night rather than during the day.
The ferry works like a floating hotel. There is a game center, 24 hour vending machines (with beer in them), karaoke room, several bars (although the bars were not open late, but that's why there are beer vending machines), a restaurant with a buffet, a movie theater, and even live entertainment. There is a public bath (onsen style) and the bathrooms were very clean. There are a few comfy couches in the lounge area and some really not so comfy ones. In all the seating areas there are lots of plugs to charge electronics as well.
Sleeping arrangements were mediocre for the two least expensive options. However, the ferry has a wide range of berths and the suites and first class cabins are quite nice and on par with the common areas. At the bargain level, two types of capsule style berths are available, one has a TV in it, the other doesn't. Inside the berths are also plugs for electronics but there isn't much room in the berth so you'll likely have to leave your bag outside your curtained off area. The least expensive room is the common room or "Japanese style" at ¥4000 per person one way. These are large tatami rooms with roughly twenty sleeping pads arranged around the room to fit as many people as possible. There are only about 5 or 6 inches of space separating the sleeping pads. There are ladies only common rooms that require a key card if you are a woman and traveling by yourself.
The English page for Taiheiyo Ferry is a list of information and a phone number to call to make reservations. To make online reservations and pay for them in advance via credit card or convenience store pay center you must use the Japanese page. Online advance booking price is half of the normal list price (when advertised). The least expensive berths are listed at ¥8000 per person, one way but booked in advance through the Japanese page, they are ¥4000 each. Booking online is by no means particularly easy. First you have to sign up for an account and be able to use katakana on your keyboard. They do not allow you to book very far in advance. About 8 weeks before the departure date is ideal. To book a return trip is really easy because you can go into your confirmed trips and click a button that says book reverse direction.
Getting to and from the ferry terminals is actually very easy. Once in Hokkaido there is a Chuo bus and Donan bus that go to JR Tomakomai Station and JR Sapporo Station. The Chuo bus is ¥1270 all the way to Sapporo from Tomakomai and is the best method to get from the ferry port to downtown Sapporo. In Sendai there is a city bus that goes from JR Nakanosakae Station (on the Senseki line) and JR Sendai Station to the ferry terminal but in the evening the timing may not be reliable. A taxi from Nakanosakae is about ¥900.