Built for the 2002 FIFA World Cup, Sapporo Dome is home to both baseball's Hokkaidō Nippon Ham Fighters (www.fighters.co.jp) and J-League soccer's Consadole Sapporo (www.consadole-sapporo.jp). The stadium switches surfaces depending on the sport being played: the Fighters play on an artificial surface; when Consadole has a match, a natural grass pitch is slid into the stadium.
The Fighters in particular have extremely boisterous crowds and a trip to the Dome is a great way to see parochial Japan in action. Check out the website for schedules. When nothing is on, tours of the dome are held on the hour from 10am to 4pm for ¥1050; reservations necessary. It's a 10-minute walk from the subway.