Sleeping, eating, drinking and shagging.
(For Redsnail delete 'Shagging', insert 'Shopping'
)
It's why we joined!
Seriously, unlikely to leave town and go very far as one maybe required to be 'contactable' so the party in Berlin is unlikely given to-days unenlightened rostering. If one did go and missed the flight back then expect the sack. Yes we often go sightseeing, sometimes in a group, sometimes on one's own or arrange a beach party with hired mini-bus, loads of food and grog etc.
Lay-overs are getting shorter and shorter so what one can actually do as opposed to would like to do is often very little except eat and sleep.
If possible always have a drink with the crew on reaching the hotel just to unwind, get to know them, exchange jokes and possibly arrange further entertainment, time permitting.