I'd like to do a couple day trips while I'm in Naples in late May -- to Capri and to Pompeii. Should I do them separately or on the same day? Should I pre-book them through an agency like Expedia? Also, please share any day trip companies that are good! Thanks so much!
I'd suggest two things: go on separate days; and just get on the train (to Pompeii) and boat (to Capri). Take your RS book and have a good time. Remember that Capri boats can be extremely crowded so try to go early and don't count on getting on the last boat back to Naples.
If buying a guide book, get Lonely Planet or DK Eyewitness.
Pompeii you get to Napoli Centrale, the main station; then downstairs to Garibaldi for one of the trains to Pompeii Scavi:
https://www.sorrentoinsider.com/en/naples-to-sorrento-train-schedule
https://www.sorrentoinsider.com/en/campania-express-train-schedule [less uncomfortable, mid-March to mid-Oct]
Licensed guides at the ruins do small group tours.
Capri on a different day you do on your own.