Haha, funny you ask... My girlfriend was going to Viterbo University this past year and I drove down there about every other weekend, but changed majors and is no longer going...
I didn't really spend any time in Onalaska, but the bar strip and along the river in LaCrosse are great places to cruise. There are also one or two pretty decent parking garages, the one I remember didn't have a guard (at least not on the weekend), was 6 stories, and you could take healside or toeside down it. There are also some super sick hills with hairpins and crazy corners in the bluffs
Most you could pretty easily get to 40+ on. I could show the places I went on Google Maps if you want...
As far as other boarders I only ever saw 2 other people on one occasion. They were riding Sector 9 completes and had no idea what Silverfish or slide gloves were. So yea I'm guessing you'll be pretty much alone.
As far as the town, like I said I didn't spend too much time in Onalaska, but LaCrosse is a total bar/college town. Onalaska seems to be much mellower, but there are college kids everywhere. There are also some places to hike and camp along the bluffs, at least on the Minnesota side, and... well its Wisconsin so don't expect too much :\
However if you did move I would honestly consider coming down to speedboard with you. So much stoke is killed for speedboarding living in the suburbs (read: cars f**king EVERYWHERE! and no good hills) Lemme know what happens with you and your wife. I'm sure if I paid for most of the gas (takes half a tank to get there here) I could get others to come along.