I think OP arrives after dark, no? If so, seeing Mt. Vesuvius would be the morning event. Down near V. Partenope they'll be more places to stroll at night, in an environment that doesn't bring up some of the insecurities, unwarranted as they may be, that travelers new to the region often feel when in Naples. But if he spends another night there, definitely go uphill for the sunset.