While Hokkaido is "wild" in the natural sense, Susukino is "wild" in the exotic sense. When the day turns to night in Sapporo, locals and tourist alike head to one place--the Susukino district. Sus'kino is filled with bright lights, restaurants, bars and clubs, massage places, and, my personal favorite, love hotels.
The restaurants start opening at 5pm. A local favorite is Ghengis Khan [or Jengis Kan]. This is a lamb that you barbeque at your own table, usually with a large beer in hand. Another speciality is ramen -- specifically Sapporo Ramen. There is a whole alley in Sapporo dedicated to ramen.
Bars run the gamut. There are discos, themed bars (my favorite is a cemetery theme), hostess bars (expensive), members-only clubs, and after-hours clubs that stay open, officially or unofficially, all night.
If you're in the mood for something more erotic, there are massage parlors. One of the most famous is Soapland, where, after a good soaping, you can pay for "extras." If you find someone, or bring your own companion (like I do), there is a Japanese adventure waiting to happen at a Sus'kino "love hotel." These are adult-oriented hotels that provide short "stays" (usually 3 hours, but sometimes 2). They are not very sleazy (aside from our typical association with sex and sleaze), partly because they are usually kept incredibly clean. They can run from the basic to the luxurious. At the most basic they provide a nice bathroom and large king-size bed (anything other than a single is somewhat rare in Japan). At the top end not only is the bath large, but a hot tub or even a private swimming pool is possible. Whether you come for an erotic adventure or just to spend the night (arrive after 11pm), these should not be missed!