Berlin - is Potsdam a good day trip?

Potsdam as day trip is easy to reach by S-Bahn line S7 or by regional trains (in both cases ABC ticket needed). Reliable connection except 1 hour on March 4 when a swan decided to land and rest on the tracks.

Unless you have a deeper interest in special sights there you can do it on your own.

Avoid using car: the Autobahn part between Berlin and Potsdam has severe actual issues, e. g. emergency bridge closings.

Definitely do a day trip! Very easy from Berlin as indicated above.

It took me two days to explore the places included on this link. (including visits inside the major palaces.) All were worthwhile. Some of the palaces, Sanssouci most likely, may require advance tickets. I had originally planned a one day trip, but liked the area so much that I returned for a second day - lots to see, plan accordingly!
https://www.introducingberlin.com/potsdam

https://www.spsg.de/en/palaces-and-gardens/information-on-annual-and-day-passes-combined-tickets-and-reduced-admissions/information-on-the-combined-ticket-sanssouci

There are local buses at the train station that will take you to the Park Sanssouci area. Or, I caught a taxi. The palaces are reasonably close to each other.

I have day-tripped to Potsdam on two separate trips. The first time, I wanted to see Cecilienhof, where the Potsdam Agreement was signed. I took the S1 to Wannsee, then took a very short bus ride and got off at Glienecker Brucke, the "Bridge of Spies". From there I walked through a pleasant neighborhood to Cecilienhof. (Google tells me that unfortunately Cecilienhof is closed for restoration. That's too bad. I found Cecilienhof absolutely fascinating. So much history!) A city bus (a minivan actually) stops at Cecilienhof and I caught that back into Potsdam, where I explored a bit before going back to Berlin.

The second time, my son and I took the S-bahn to Potsdam and caught the bus out to Schloss Sanssouci from the train station. We toured the Schloss, but it was too cold to do much more that day. We could see our breath inside the Schloss!

We were closer to a train station with an earlier departure back to Potsdam HBF from Sanssouci, so we took that instead of the bus.

So yes, it's very easily done on your own. Public transport is plentiful. I used Google Maps for navigation, both with public transit and walking.

An ABC zone ticket will cover your trip to Potsdam on the S-Bahn as well as the airport, if you're traveling to/from it and are interested in a multi-day pass.

There are local buses at the train station that will take you to the
Park Sanssouci area

Or, take one of the regional trains that continue beyond Potsdam Hbf and alight at the station Potsdam Park Sanssouci, and the park is just across the road.

Thank you to all of you for your responses. We thought about spending the extra time in Berlin because we love the city. However, all of you have convinced us we do need to visit Potsdam. I appreciate all of you.

We took a day to walk through Potsdam. Took a train to the Park Sanssouci area. We walked through the gardens past all the Schlosses. We didn't take time to go inside any.
From there, we walked through the Old Town and the Dutch Quarter, where we found a picnic lunch. It was a warm day and it felt like we may have spent too much time in the sun.
Then we walked to the Babelsberg Park for lunch, sunbathing, and swimming in Tiefer See. Turned out it was a clothing optional beach, common in East Germany. Afterwards, we took an S-bahn back to our lodging.

I had a similar experience to Leonard's above. Not only the beach, but the parks are used for (discreet) nude sunbathing. Or, were when I strolled through en route to the Glienicke Bridge a few years back.

I was glad that I was warned at my Berlin hotel that the mixed-gender sauna was "a nude sauna".