Although we preferred to go to Pompeii from Sorrento as a day trip, many posters here have used the luggage check counter at the main entrance to Pompeii, in order to make that stop part of a travel day. While agree that it is better to go in a cooler part of the day, I've never seen any comments about the luggage storage that were not positive. Pompeii needs a minimum of two hours, and ideally perhaps 4 hours. There is a large cafe inside the ancient site, which is not true, for example, at Herculaneum.
Because you did not mention either Capri or the Amalfi Coast, it is hard to be sure which is better for an all-day excursion. Both have time-consuming travel, and they are very different from each other. Honestly, the big minus about Positano for me was the high-priced European boutique shopping storefronts, in which I have little interest. But it is a very famous playground of the rich. If I could only do one, I would do Capri. But if you go to Capri without a plan, you will wander around and miss the best sights. I don't depend on a newsboard to point out individual sights for me.
You could use the Search box to read about the Blue Grotto (or not to go ... ) since it such a frequent topic. We thought it would take too much time, so we skipped it on both of our one-day stops on Capri. (The second visit was on a small cruise ship, not a ferry ride from Sorrento.)