I'm bicycling, so I get to go where and when I want!
In this case don't just visit Karlovy Vary but also Loket, Cheb, Marianske Lazne and... ahm... I can't spell it in Czech... Franzensbad. This is a wonderful area of the Czech Republic. Beautiful nature, posh 19th century spa towns, a interesting historic town and a picturesque medieval castle.
The Czech Republic is much more than just Prague and CK. Use the map function of www.czecot.com to find out what can see on your way.
I think so with Plzen if you're into the history in 1945
Many places tourists visit in the Czech Republic are interesting in this respect, but most people are just obsessed with the Nazis and don't care that Cesky Crumkov for example is as fake as Disneyland since it was ethnically cleansed in 1945, or that Lidice wasn't the biggest massacre of the 20th century in Czechia (that was Postoloprty) etc...