I usually visit the place (often with other C/C or pilots) if I don't know it or return to places I liked if I have already been there. Also go for shopping (guilty!
) or movies if it's a long layover. When I'm really tired I stay in my room and read or watch TV.
If there is an internet connection I pprune!
The crew usually meet for dinner at night and often the Capt pays for a drink or a bottle of wine.