the best day trip from Prague

There is more to see in Kutna Hora than just ossuary. Silver mines, Santa Barbara Cathedral and just streets and squares of this medieval town. Or Terezin. It depends what attracts you more. Or one of three castles: Karlstejn, Krivoklat, Konopiste. You can read more about it in Rick Steves book: Prague & the Czech Republic.

I would visit Kutna Hora. The town offers much more than just the bone church, as Ilja already wrote. And he forgot to mention the spectacular panorama and some sights for historically interested people like the Sedlec monastery (a short walk from the bone church), with some great paintings of the Bohemian/Silesian baroque.
BTW: a day trip to Dresden in Germany is also possible (2:15h), in case you want to see some world-class museums like the Green Vaults or the armory.

We have visted Kunta Hora but not Terezin. Kunta Hora was an easy train ride from Prague. As I recall it was a short, maybe 15 minute walk from the station to the ossuary. Despite our efforts to stretch it out, we saw the chapel in less time than it took us to walk there. Interesting indeed, but small. A shuttle was running into the city. We walked. A quite long, unattractive walk. The city itself has sights to offer and is nice enough. However, we had nearly a week in Prague; had I taken one day out of three for Kunta Hora I daresay I would have been honked. If you have never toured a concentration camp and are traveling with adult children, I would opt for that. We have, and they are among the most moving and unforgettable experiences we have known. There will be plenty of cathedrals and chapels elsewhere.

I noticed that more people write Kunta Hora instead of Kutna Hora. I advise against that. Kunta sounds very similar to the certain word in Czech language which is considered vulgar. President Zeman got himself in trouble when he used that word several times when he spoke about Pussy Riot in Russia.

I'm still laughing.

Why don't you look into Plzen instead of where all the tourists go. Its a nice town to wander around in and it has a world famous brewery. There's history there, great food, and a university.

been to all the places mentioned.
Terezin is about an hour by bus from Prague and you need at least 4 hours there to get round the sites as they are spread about quite a bit.The focus is mainly on the period the fortress was used as a concentration camp but there really is much more to it that that but there is some scant info on it .
Kutna Hora (I used to spell it wrong too) is also spread out, the Bone church is about a 10 minute walk from the train station and the main town about another 25-30 minutes walk from there (you can get a mini bus from the Bone church ) just ask about it inside the church but they usually wait till they have 3 or more folk, last time I Used it (November last year) there was a group us 5 of us, can't remember the price but nothing much, the mini bus stops just above St Barbora's catherdral so its all down hill from there, really loads to see in the town it is quite lovely.
Plzen is great I have a have visited many times, mainly for the brewery and brewing museum.............well actually just for the brewery and brewing museum but nothing wrong with that., easy train ride from Prague and the train stops just a short walk from the brewery.

whilst I am on a roll have a look a Tabor, nice place about 90 minutes from Prague ,a former Hussite enclave it has a most unusual layout , a nice little tower and medieval museum, and underground tour as well as a Hussite museum , a nice church and a very pretty little square ,not many tourists go here and it is one of the friendliest places I have visited in the CR.

Thanks for the correction Ilja. Not an error one wants to make.