I do not know what it is like now, but I visited Walthers a few years ago and it was basically an order counter, a couple of displays, and an HO layout. Nothing I would drive from Waukesha to see if time was tight. On the other hand, Sommerfeld's in Butler, a suburb, has a pretty nice 0 gauge showroom. It's just off Highway 45, not too far from Waukesha. You can check out their website: Sommerfeld's The About page has a picture of the inside of the store and there's a map. There's also Hiawatha Hobbies in Pewaukee, which is just up the road from Waukesha. Not as big as Sommerfeld's but they have some interesting stuff. I'd skip Walthers and try the other two, especially if time is a consideration.
There's also a beautiful old stone Milwaukee Road station in Oconomowoc with an excellent restaurant if you have time to try it for lunch or dinner.