Zao Sumikawa is a small snow park two hours south of Sendai in the Ou Mountains that is very focused on two things. First and foremost are snow monster tours. This is the only resort on the Miyagi side of the Zao area that offers a snow monster site seeing package. Second, Sumikawa is a snowboarder's dream and nightmare at the same time.
For ¥6700 you can get round trip bus fare and pay for the entire tour of the snow monster area in Sumikawa. The Sumikawa homepage is all in Japanese but I went to the Tourist Information center in Sendai station and they were able to book my bus tickets for me. The tour must be booked at least one day in advance. Once at the mountain, you can climb into one of three or four massive snow crawling tractors and be on your way. From the top you will be able to see the famed snow monsters of Mt. Zao and on a clear day even see Matsushima.
If package tours are not your thing and you are interested in skiing or snowboarding, ¥4800 gets you an all day lift ticket and round trip bus fare from Izumi Chuo, Sendai, or Nagamachi stations. The bus arrives at Sumikawa around 10:30 after making stops else where around the Zao-machi area. The return trip leaves at 3:30PM and that seems like a short time to be boarding or skiing but the mountain is small so you should be able to hit all the runs. Sumikawa's neighbor, Zao Eboshi, is a better overall value for the same price including round trip bus fare. Eboshi is a larger resort with more variety and less hiking overall. North of Sendai is also Spring Valley which is a good value as well.
Sumikawa is a snowboarders dream if you really like the park elements of snowboarding. There is a massive collection of terrain features both natural and man made to help you on your way to becoming an awesome snowboard trick master. However, if you like simple riding, Sumikawa is a nightmare. Most of the runs are the same cat tracks they use to take the tours up the mountain for snow monsters viewing. Out of the 9 runs on the mountain the ones at the top are the only ones with any grade. The elevation change is basically nil, only about 300 meters total, and all of that concentrated at the peak. Of the challenging steep areas at the top of the resort, only one can be reached without hiking. As a snowboarder I found maintaining speed extremely problematic on the lower parts of the mountain.
To be absolutely fair, if the snow is in good condition, Sumikawa does have one major benefit. There are virtually no boundary lines anywhere in Sumikawa. You can go down any gully, through any copse of trees, down any slope and around the next. Sadly, when I visited there were some serious thaw-freeze conditions where the snow melted in previous days and then froze to solid ice, which is not very fun to board or ski on. Had the snow been in decent condition the top of the resort would have offered some excellent back country style boarding. However since there are only 3 lifts in the entire resort, be prepared if you go off the main runs, to do an immense amount of hiking.
Before you head out to Sumikawa, check the snow conditions. If the conditions are anything but fresh powder snow I would think about a different resort. If there is powder snow, get ready for an awesome back country feel at a small resort. Regardless of the snow conditions the snowboard park is excellent or if you simply want to see the snow monsters without going all the way to Yamagata and Zao Onsen, Sumikawa is a great place to go.