If the day's weather forecast is grim, why not spend your day at Museo Nazionale Romano: Terme di Diocleziano? You'll be fascinated by just how many interesting facts you can learn about the past here.
This collection of historical artifacts in Rome City Centre is popular with travelers, so make a spot for it on your schedule. Don't stop there, though. There are plenty more spots waiting for you to appreciate after National Museum of Rome - Baths of Diocletian. St. Peter's Basilica, Roman Forum, and Piazza Navona are close by.