Rome is centrally located in Italy and has the best rail connections. From Rome it is a 4 hour high speed train ride to Venice each way. That's a lot of travel for a day visit. I would suggest an overnight stay. A very nice hotel near the train station is the Hotel Olimpia - https://www.hotel-olimpia.com/48 . Both Florence and Naples can be done as day trips as they are only a little over 1 hour from Rome by high speed rail. A good hotel that is near the train station and Metro in Rome is The Hotel Massimo d'Azeglio - https://www.romehoteldazeglio.it/en/193 .