Have about 4-5 days in Prague and wanted to explore a day trip option. Going in late Oct early Nov, so looking for options. available then. Will likely have a 3 country Flexipass from Eurail so can use one day for this (Doing Budapest/Vienna/Prague over 2 weeks). Thanks.

Cesky Krumlov is a not to miss place, castle and all. If you will put "day trips Prague" in the search box you will find many suggestions to ponder. Have a good trip.

From Prague, the two day trips that I recommend if you have the time are C. Krumlov and Terezin.

But, with 4-5 days in Prague, you will only be able to do one at most. Prague is so wonderful, you will find tooooo much to do.

Since you are doing Budapest/Vienna/Prague over a two week period... I'd do an overnight stop in C. Krumlov between Vienna and Prague.

The castle and baroque theater in C. Krumlov are wonderful. The theater is one of the 2 (4?) remaining baroque theaters in the world and well worth a visit.

I did the reverse trip... 5 nights Prague, 1 C. Krumlov, 4 Vienna and 4 Budapest.

Cesky Krumlov is not really a day trip. It takes 4 hours by train (3 hours by the boring bus). I think Cesky Krumlov is particularly special at night so if you can, have an overnight stay. No one has mentioned Kutna Hora yet, a wonderful day trip, being just an hour east of Prague and well connected by direct trains.
http://www.jeremytaylor.eu/cesky_krumlov_photos.htm
http://www.jeremytaylor.eu/kutna_hora_1.htm

I agree with Jeremy,Kutna Hora is a lovely place and very easy to reach from Prague in about an hour or so.