Very easy by train or bus. Tram/train tickets are sold in front of ship, next to tram top. Leave the tram after the left corner; this is the station. Pompei Scavi is stop 18. Guides are available and useful. We were back before lunch.
Easy, with a free shuttle to town. Station sells BIRG tickets 12 euro, for train and Rome transport. Train takes 1 hour, with a stop at the Vatican or town. There is a lot top see in Rome but it is hot. When we entered the metro two girls tried to steel so watch out. We returned from Termini station; it is a long walk from the metro to platform 29 on the left. (15 minutes)