By car?

I'd include a few days in Prague definitely, and a visit to the nearby Karlstejn castle. In Germany Berlin is obviously great, but a probably too far from your route. Munich is easier for you and nice for a day or so; and the Neuschwannstein (castle that was the model for the Disneyworld one, 1.5 hours from Munich) is stunning.

In Austria you may want to spend a day in Salzburg (lovely town and birthplace of Mozart). In Poland apparently Krakow is very nice but I've never been there.

Just saw your travel plan; seems like you have most of my advice covered already. I'd consider Nuremberg instead of Vienna, since I wasn't impressed by it. Good idea to add Brno; it's very nice.

Thanks Edward, Brno is now in my route. I am gonna stay there. But still I need to know some more places where I can stay and visit. Especially in Poland.

My Updated Travel Plan 

http://g.co/maps/aauer

Still wondering about the route shall I take to come back?  

I'm not an expert on Poland I'm afraid; been there twice and only in Wroclav (which is OK but not worth the detour). Krakow and Washava are worth 2-3 days each, from what I've been told.