I work for a travel company in England with Germany and Austria as two of my destination of responsibility.
Germany - As already mentioned you need to try and get to Berlin, it is an amazing place. Going in a couple of weeks gets you into May, so you could find some of the beach bars could already be open along the river. Deck chairs and cools music whilst sipping a drink in the middle of a big city is something else.
The easiest way to see all of the city is to get a pass for the hop on hop off bus. They are bright yellow and go to 16 of the key places to see around the city. As you have probably guessed by the name you can get on and off as many times as you like. If you stay on and just go round the city it takes approx 2 hours.
The Berlin Welcome card is also a good investment, it goves you free transportation around the city on the trams or trains plus loads of discounts and free entry for museums, restaurants etc.
If you want to see one of the last few remaining sections of the wall head for the East side gallery. It is the longest remaining stretch of the wall and has been turned into an art gallery. They basically got a load of artists from around the world to paint on different sections of the wall. Last time I was there Playboy were doing a shoot
If you are looking for night life it is pretty easy to find but Berlin is split into different districts and each district has it's own bars and clubs, it is not like you can go downtown and they are all in one place. You need to do a bit of digging before you go on this one.
For the other German cities, Frankfurt is okay but your not going to miss much if you go elsewhere. Cologne is really nice, if you arrive by train you step out of the station and the huge cathedral is right in front of you, it is quite a welcome. Munich is also really nice and you can visit the Neuschwanstein castle from here, as mentioned by Knalus. If you don't have a car it can be reached by train, it is a short walk from the station but a bit of a walk to the top of the hill where the castle is.
Oh yes one last thing, getting around germany by Train is dead easy. I tend to use DB Bhan (
http://www.bahn.de/i/view/USA/en/index.shtml), they have lots of inter city trains and over night sleepers if going a bit further. Really well priced as well.
Next stop Austria