Yeah, Arch and Budweiser are good in STL. I would also recommend they get pretzels (or pretzel dogs?) at Gus's Pretzels, which is literally right across the street from the brewery: http://www.guspretzels.com/

Also, anything in Forest Park is grand and free, particularly the Zoo and the Art Museum, but it might be a bit cold for the Zoo. There are also some neat old houses near the brewery and gus's. And, if they like Native American stuff, Cahokia Mounds right across the river is a great stop.