The Charles Bridge. Nowhere else in Western Europe will you find a denser concentration of street vendors peddling to the stupidity that seems to lurk within all of us tourists.
Charles Bridge is a great spot to hang out and take in the view, but it's also Tourist Trap Central. The bridge is crammed with people selling souvenirs. Try it early in the morning.
As mentioned above, Cesky Krumlov can be quite the tourist trap. But take a look below - I think it's worth it, especially if you go off-season like I did.