Back when I was in coffee, I visited with the guys from Alterra (before they sold to Mars). The old pumphouse that they converted into a coffeehouse (with balcony seating) was the most uniquely designed and situated coffeehouse I'd ever visited.
Oddly, my 2nd wife's parents lived in Cedarburg, so I was up there quite a bit. But they never wanted to go "to the city" despite much prodding. Wife's stepdad was a columnist for the Sentinel and still, he'd rather take us on rides to places where he wanted to buy a tree farm than go downtown. We never went out to eat on any visit. I, OTOH, took him straight to Pepe's for a white clam pizza when he visited. Blew his mind.
You forgot to mention that the lake water is really freaking cold even in summer.
That really is a unique place, and still open. Not only as a coffee shop, but as a pumping station for the city!
Coincidentally, I actually live in Cedarburg. I think I even know who you're talking about if he's now retired? The thing about Cedarburg is since they have that whole Hallmark movie city thing and being sort of a touristy day drive/road trip destination thing going on you have a certain population that was born, raised, and lived there forever that has become somewhat... ummm... provincial we'll call them. If this group finds out you moved from Milwaukee, they'll assume you're part of what is ruining the charm of the city. I'm surprised they didn't at least take you out to eat IN Cedarburg... there's a few REALLY good restaurants. But again, if your in-laws are like the rest of this group, their minds would probably be blown by many places outside their bubble.
Also, I didn't forget the lake temps...
Just kidding, I meant to mention it. Most summers though you'll at least get a good second half of the season where it's not bad. lol