Eating at night in Niseko is a rip off
Head over to Kuchan (there's a free bus that takes you there and back)
just wait outside the 7/11 store on the main street at Niseko, bus every 30 mins
heaps of izakayas in kutchan - jap pubs
its worth visiting as many as you can
watch out for the junior yakuza (mafia) that speaks pretty good english, trying to get you to buy stuff or go to onsens (spas), its a front

As somenone has mentioned, look out for the little vans on the street
you can eat cheap for 200-500 yen

dont think you'll get to watch the sumo as its on selected months of the year, and jan is one of them (and its specific day only)

...oh if anyone need snowboarding gear (clothes) let me know
jackets, pants, etc
wont be snowboarding until junior gets older