There is no reason that an instructional flight cannot also be a recreational flight.

The key is that the flight must have actual training value (ex a required XC) and both CFI and student need to have a shared purpose at the recreational destination: Lunch, skiing, partying, visiting family, etc. The CFI would need to pay his own way on the ground (exception would be the tradition that the student buys lunch/dinner...but not hotel, beer, lift ticket, hookers, etc)

If the student is going to a business meeting or family get together, and is paying for the CFI to hang out in a hotel, that would sound more like a charter flight. Even so you could probably get away with it once...say on his long XC.

It also helps if there are better transportation modes available...ie LA to Vegas airlines would be cheaper and faster than GA.

Where people get in trouble is they keep doing the same thing again and again, and log it is "XC dual-given". The "student" usually doesn't even have a third class medical or logbook...