We will be driving from Munich and would like to stay the last night somewhere that is interesting and picturesque. It should be no more than 2 hours from the airport as our flight is at 2pm. We thought of Plzen or Marianski Lazne or Loket? Any advice?
Marianske Lazne and Loket are picturesque. Plzen less so. If you are into beer than Plzen could be interesting to you. The largest and most famous brewery in the Czech Republic is there and you can take a tour there. It's a birthplace of a pilsener beer. Plzen in German is Pilsen. Marianske Lazne is the second largest spa in C.R. and IMHO the most beautiful. We stayed in hotel Koliba which is completely in the woods (from your room you can watch deer grazing around) and still only about 5 to 10 minutes walk to the main colonade. A lot of parks and walking trails. Loket somehow reminds me of not disneyfied Cesky Krumlov. Castle and town on the hill with the river flowing around on three sides. If you decide for Loket you will be only about 5 miles from Karlovy Vary the largest spa in C.R. and also beautiful.
Plzen, Marianski Lazne and Loket all sound good. Loket is very small though, you have seen the town in 1 or 2 hours. Marianske Lazne is a smaller and more relaxed version of Carlsbad, that's why I like it more. Plzen is OK, but I'm not a great fan of the city. I would prefer Cheb, a historically important town with a nice, preserved centre. You can't do much wrong in this northeastern part of the country, all the famous spa towns are there for a reason.
I think you meant northwestern part of the country.
Looks like you've had some relevant advice. Hope it's a great adventure!