Day trips from Berlin.

Others besides me can comment further, but I believe Dresden is probably too far for a daytrip. Likewise, as much as I enjoyed Meißen, particularly it's fantastic castle, it would be more suited as a daytrip from Dresden than Berlin. I haven't visited Saxon Switzerland, but same thing probably- see it from Dresden, not Berlin.

I'm not sure how long it would take to reach by public transportation from Berlin, but I enjoyed Wörlitzer Park (and the adjacent village), near Dessau. Dessau itself, though, is skippable.

Dresden is just over two hours by the fastest express trains. That's a bit too far for a day trip for me, but it's up to you.

All places you mentioned, except the Spreewald, are quite far away from Berlin by train. It takes ~3 hours to get to Meissen and Saxon Switzerland, and even to Dresden there is only one relatively fast train (2h) every 2 hours (fun fact: the connection to Dresden is slower than before WW2). So, the Spreewald is perfect for a day trip, Dresden is doable (only the old town + museums, not Pillnitz), but I wouldn't recommend Meissen and Saxon Switzerland.
For a medieval, preserved town I recommend Tangermünde. The town is completely unknown to most foreign tourists, and will never appear in Rick Steves books, but offers impressive fortifications and a very nice old town. It takes 1:30h to get there. Even closer (0:40) is Wittenberg, which offers a nice, small old town and many places related to the Protestant Reformation, like Luthers (with preserved interior) or Melanchthons house (interior).
If you really don't mind to travel for more than 2 hours I recommend to see Erfurt (2:30), the capital of Thuringia. The city offers one of the biggest and best preserved old towns of Germany. Rick Steves calls Erfurt a "untouristy Rothenburg".

Regaring passes: you have to figure it out for yourself. The more discounts you use, the more cost-effective they become.

Thanks a lot for your suggestions! Lots of new ideas for future trips. We've decided to do the shorter trips instead and will do Spreewald and Wittenburg or Leipzig.
We'll also get the 7 day weekly card and 3 day museum pass.
Val

I enjoyed the Bastei Bridge as a day-trip from Dresden, and Meissen was only a little more than an hour by S-Bahn from Kurort Rathen (the stop for the Bastei Bridge). [I went to Meissen on the spur of the moment--unprepared, not much in the RS book about it--ran into an unhelpful TI--and did not enjoy my time there. If I went again, I would be prepared and probably love it.] The train I took from Dresden to Berlin was 2.5 hours, but I stayed in Dresden (Etap/Ibis Budget Dresden--loved it!) and day-tripped from there (Bastei, Meissen, Goerlitz, Bautzen) before returning to Berlin, so I can't imagine any of it as a day-trip from Berlin. Have a great trip!