Plzen, historically known by its German name Pilsen, is a wonderful compact walkable town centered on a picturesque square. It's an easy and scenic train trip from Prague, with tickets costing just a couple dollars. The tourist crush in Prague can get a bit overwhelming, so even though Plzen is not really off the beaten path, it feels like an authentic relief not to be bombarded by crystal and trdelnik shops at every turn.
I'm a big fan of craft beer, but giant Pilsener Urquell does make a very good product. The tour is very well done, showing how it's made today and explaining the history.