Day trips from Rome

Naples and Pompeii are doable in a day BUT it is a very long day. I would personally save it for another trip. Places like Ostia Antica, Tivoli, Hadrian villa make for good day trips. Ostia Antica has some similarities to Pompeii but is not a substitute as some might suggest.

Orvieto can be done in a day. The Castelli Romani (hill towns near Rome) can also be worthwhile.

I was in Rome in late August. Did both Ostia Antica, and Tivoli as day trips from Rome with friends who live there. Highly recommend both to provide context to Roman life. Save Naples for another day!

A travel companion wanted to see Pompeii from Rome. It's a four-hour bus or train ride. If you take the train from Rome, you go to Naples and then change trains to Pompeii. If you book a tour, you only get two hours at the excavations. We booked an overnight tour thinking we'd get more time at the excavations, but no, we did not. Save your time and energy and see Ostia Antica, which is right outside of Rome. It's smaller than Pompeii, but gives you just as good a flavor of an ancient city. In Ostia you will see larger mosaics in situ than at Pompeii, and some buildings preserved to a greater height. I recommend seeing Naples and Pompeii on a separate trip to southern Italy.

I haven't gotten to Hadrian's Villa yet, but its discovery had similar effect to Herculaneum and Pompeii on Western interest in things Neoclassical. Check the Tivoli hours carefully. It's hard to do both Tivoli and Hadrian's Villa in the same day, but perhaps with a car?