‘d probably head down to Naples for a night or two and see Pompeii.
definitely worth doing. that and Herculaneum. You can also drive 2/3 of the way up Vesuvius, park up, and walk right up to the crater rim. done that. had a week in Sorrento, lovely place.
Capri is nice but I’m not sure it’s worth the boat trip TBH, unless of course you like boat trips.
re Rome – if you want to see the Colosseum, better to queue for tickets round the corner at the Forum, rather than at the Colosseum itself.
If you want to see the Sistine Chapel, definitely worth booking online beforehand, as mentioned above. we didn’t know that, never got near it in the short time we had available, so we went to the Basilica instead. Awesome architecture & artwork but I didn’t get any kind of spiritual feeling from the place, compared with say Chartres cathedral (or even Wakefield!)
some pics here: https://www.flickr.com/photos/johnhooper_drum/sets/72157645170034827/