I went for a few days in late October last year (stayed in Pompei), had a fantastic time, and the weather was great - it got chilly by late evening but I can't remember any rain and the days were largely very sunny with temps in the mid 20s. I did Pompeii, Herculaneum and went to Capri - and I definitely wouldn't have wanted it any hotter when I was trekking up to Villa Jovis!
I love ancient ruins so unsurprisingly I recommend Pompeii, Herculaneum, Villa Oplontis etc- they all have a different feel to them (IMHO). Pompei is very exposed and busy, and also massive, so coming early is a must if you want to see lots of it. Guidebooks are only at the main entrance, but if you hate Guidebooks the queue at the Ampitheatre entrance is usually much shorter, and has a big display of casts outside. Herculaneum was less busy but no less fascinating.
The full price ticket for Pompeii is around €15 from memory and the Circumvesuviana is cheap and frequent so don't pay £45pp!
I loved Capri - it was very busy in Capri and Anacapri themselves, but I took the chairlift up Mt Solaro and walked back down - and I had the path completely to myself all the way! Stunning views, very warm but not hot, combined with absolute peace and quiet, it was bliss! I walked to Villa Jovis (one of Tiberius Villas) and did the audioguide tour, walked back and again there were barely any other people there, the views were incredible and I largely had peace and quiet.