For a day trip, my basic criteria is "Can I do the one way trip in 1.5 hours" This makes for 3 flying hours there and back which I find quite sufficient for one day.
This gives me fairly good access to some of the coastal towns and the wine areas down south. Can fly to an estate for lunch and watch everyone else getting slightly sozzled.
Of course, if its bumpy on the way back, I usually get the last laugh.
Anything over that and it becomes an overnighter.
Mind you, I do live in a place with plenty of space so it makes it a bit more difficult for me.
