Underneath the Brewery Museum lies a whole maze of city streets--in some places up to three levels deep. While there isn't so much to see down there, they've done a great job adding miscellaneous bits and pieces of history throughout the walk. And the best part? They've got a free app (for both iPhones and Androids) that allows you to follow along while your guide addresses the group in Czech! It's well narrated and offers a great glimpse into the city's history, both above and below the streets.
Be sure to book ahead as tours fill up fast and if you want to take pictures, you'll need to get a photography pass, available where you buy tickets (cost: 100 Czech Crowns).