I'd be remiss if I didn't mention my guildmate, Heather, 's store Buttons & Bolts in Salem, WI. It is southeast of Madison, near the Illinois border & has all sorts of adorable little trinkets & lovely, quality fabrics. It's near Kenosha, which has both tons of places to eat as well as a fantastic designer outlet mall (Salem, WI is a very, very small town).
Not sure how you will be traveling, straight south of Salem/Kenosha is Fox River Grove, IL. Pieceful Gathering is a shop that only stocks reproduction fabric from the Civil War & 1930's-50's (mainly Civil War, but a good selection of both). It is a cute little 2 story historic house that they converted to their shop & I absolutely adore it. You wouldn't believe how many fabrics they have crammed in there (all nicely organized). They also carry lovely wool & felted wool fabrics and a good sized collection of Primitive Gatherings wool applique patterns (Primitive Gatherings is in Menasha, WI and are also great from what I can tell, but they're 2 hours NE of Madison so you're better off with the limited selection at Pieceful Gathering) and a small but decent selection of pearle cotton. What I love about them most besides the fabrics is that they carry a bunch of colors of the 200m (small) Aurifil for my applique work in a few dozen of the colors of their fabrics, plus some basic neutral YLI hand quilting thread.
Right now they have a Shop Hop Promotion that I believe is running at both shops. If you sign their guest book you get a discount on pretty much your entire order (trying to remember what it is -- 10%, I think). I know PG is running it. Also, if you have a specific request for PG, you can call ahead & they will try to order it for you so it's ready for your trip, or can ship it to you if you order it while at their store (you pay shipping).
Sounds like a really fun vacation! Madison is a great city and you'll get to drive through many beautiful wooded areas & lakes on your way up there.