May is a nice time to visit Germany - lots of colourful events going on at this time. Carnival of Cultures in Berlin (last weekend of May), Rhein in Flammen in Bonn (the first Saturday of May), Hafengeburtstag in Hamburg ... Check out the event calendar for the countries you're going to visit
Besides the fact that Berlin is simply beautiful in May, I can only recommend to take the time to visit some historical places, we have a lot here
If you are willing to visit a museum/sight of the Berlin Wall, I would recommend you to go to the East Side Gallery. Here, you have a lot of old Wall pieces with paintings which you can walk around, it is located a the site of the Spress (teh river flowing through the city), so it's very nice on a warm day. And also, I lately have been to the new museum located just there. -snip-
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I recommend Góry Stołowe in Poland and Adrspach in Czech Republic.
Then you can go to Wrocław, PL.
Hope it helps:-)