Which of these would be the best day trip in terms of time and interest. (I love historical sights):

Quedlingburg
Stettin Poland
Dresden
Potsdam
Gorlitz

Thanks

Potsdam for sure, Dresden also, but maybe as an overnight.

Also, Berlin is so spread out, that there's more than enough ground to cover.

Make sure you visit the huge Red Army memorial site in Treptower Park.

Definitely Potsdam, but also Cottbus with its nearby castle and park:

https://flic.kr/p/fme8fG
and a nice central square containing a former pharmacy turned into a museum that covers several centuries of pharmaceutical practices:

https://flic.kr/p/fkZa9t

Quedlinburg is too far away for a day trip as is Görlitz (assuming your mode of travel is train). Stettin isn't exactly around the corner either as well as Dresden which really isn't a day trip option. It is its own destination between Berlin and Prague and not just a place you stop by for a day.

So yeah, Potsdam is always your first choice for one or even two day trips because it is so very close. The second easy destination is the old bishop city which gave the country surrounding Berlin its name, Brandenburg an der Havel. And third would be Wittenberg and its Luther sites.
If you are willing to buy tickets for fast trains, Leipzig is also an option though again it is rather a destination for multiple days.

Görlitz and surrounding area make a nice day trip from Dresden but it's too far away from Berlin. While near Görlitz you can also take the steam train at Oybin & do some light or moderate hiking.

There are much more historical sights in the area.
But when will you do this trip:
next year or in late automn 2020 already?

Glad to hear things remain open. I just received an email from the Platzl Hotel in Munich that they are closing until the end of April