Castel Nuovo - the new castle to replace the old one. I didn't make it to the old castle but I'm outside with the catapult rocks. Built in 1282, it's old. Fun to wander around for a bit.
Herculaneum - empty streets from a city that was destroyed by Vesuvius. These ruins are fun to walk since you can go in all the houses and streets. It's smaller than Pompei and worth a couple of hours.
Below is part of Pompei. There's about 4 hours of wandering if you want to do it all. I recommend two hours for most people as you start to get sensory overload and much of the ruins are blocked off.