Relaxing, shopping, washing up the gear,
that's what I usually do.
Going around to see local attraction is a good idea too, although it seemed that I'd never had enough time, I used more than half of 24 hours for sleeping and having looong breakfast and loooong dinner. Two hours for airport journey, preparation etc.
Left me a bit for local shopping, another loooooooooong meal and swimming pool session.
Swimming with mask (fold the nose part), and when I came back to city, I swam in swimming pool with googles and tried to clear the fog by letting the water went in from google top and blew the nose useless.