Favorite day trips from Prague

Lidice would be a half day trip; pair it with a visit to the Saints Cyril and Methodius Church in Prague.

I visited Olomouc as a day trip from Prague. It's a two hour train ride each way. The Tourist Info center offered a walking tour which was very informative.

what are your interests. I go to places like Plzen and Ceske Budejovice for the breweries and micro breweries as well as take in some of the history,been to both places many times and there is always something to see that I have not seen before.
Lidice is a very good suggestion the museum there is truly a must visit place and don't forget to visit the house no 116 in the area that was built for the surviving people of Lidice just after the war.https://www.lidice-memorial.cz/en/memorial/one-family-house-no-116/
It is about a 10 minute or so walk from the main site.
Kutna Hora is a nice place with lots to do for a day but there is a lot of walking and it is built on quite a steep hill.
Karlova Vary a Spa town is worth visiting and if you have the time you can arrange spa treatments or just wander and enjoy the place, take the funicular to the Diana tower for the views and there is a nice restaurant at the tower.
Podebrady is a small spa town about an hour or so from Prague by train, nothing like Karlova Vary but a very pleasant place for a few hours and a fraction of the tourists.
Karlstejn and Krivoklat castles can be done as a day trip if you plan it correctly and there are tour companies that do Nizbor glass factory and Krivoklat castle as a day trip.
lots of options about really depends if you want to have a group tour organising things for you or DIY tours.

We also enjoyed Olomouc, but also Brno. A visit to Konopiště Castle (home of Archduke Franz Fernidad) also makes a good day trip, easily done by train as the castle is an easy walking distance from the station.

We are WWII buffs, so a trip/tour to Terezin with Pavel Batel was a part of our itinerary.

@ TravelBug79....If it's WW2 material you're after, I would suggest the Army and Tank Museum in Prague.

It was in Czechoslovakia in the inter-war years where the biggest armaments factories in Central Europe were located, the Skoda works in Pilsen. That museum in Prague might be of interest to you.