Interesting and non-standard day trips from Berlin

Just wondering if any of you have good suggestions for sights and towns worth visiting within a couple-few hours drive from Berlin (or if further afield, worthy of a one-night stay). We're nearing the end of an extremely enjoyable tenure in the city and want to make the most of the 'spring' and venture out into the countryside. Particularly fancy visiting Poland again- any nice spots just over the border? Some places we've already done, by way of example, are:
Potsdam (few times, for the palace and christmas market), Dessau (bauhaus architecture), overnights in Weimar and Dresden and Wroclaw, Framus guitar museum in Markneukirchen, Frankfurt (Oder)

Try Meißen & Festung Konigstein near Dresden if you ddn't see them while you were there,

Spreewald (Burg)

Lutherstadt Wittenberg

Rügen island on the Baltic

Prague is only 3 hours away by train.

Görlitz ... Bautzen ... Zittau

Saxon Switzerland is a must. Even if you're not normally the most active people, it's beautiful there just to relax. Would recommend (Ferdinand's Homestay for your overnight stay.
You could also combine this with a trip to Prague as it lies on Berlin-Dresden-Prague train-line.
Leipzig is also worth a visit.

Very good idea for a thread, thanks!

Rheinsberg - quaint artistic town in Brandenburg about 2 hours from Berlin

www.rheinsberg.de

Couple of highlights:
Rheinsberg Palace (neoclassical style, built by Friederick the Great's brother Prince Henry of Russia)

River cruise (town center is right on the water you can easily pick up a boat at any hour)

Nice cafes

If you are a fan of Kurt Tucholsky dont miss a museum dedicated to his work, right near the Schloss
http://rheinsberg.de/tucholsky/

Good visitor's center which can plug you into concerts, readings etc.

would also recommend sächsische schweiz and particularly ferdninands homestay. great hostel, the two guys there are sound and the area is really nice. free bikes at the hostel!

Wolf's social club in the Ruhr, sounds like fun.