+1 vote for Berlin. I go there a couple of times a year with my family, really enjoying the city-life. I mostly stay in the old east Berlin, enjoying the parks and Zoologischer Garten and such, but I've been to most of the tourist trap things several times, so I'm hardly the typical tourist.

You'll want to see the Berliner Dom (big church), Brandenburger Tor (newly renovated and quite inspiring portal), Fernsehturm (TV tower with a rotating restaurant). Also, a tour on bus 100 takes you past a great deal of sights. All options available within walking distance. Checkpoint Charlie museum was kinda neat.

Potzdamer Platz is pretty, but I didn't find it very interesting per se - it's just a bunch of shops, mostly - apple store, sony store, lego store, tourist-priced café, bleh

I've never taken the time to visit their government building (Bundestag) - there's always been a huge line waiting outside, so we haven't bothered.