On the plane, my usual habit has been to read or sleep, regardless of whether I'm travelling for business or leisure. I do find that some of the travel time when it's difficult to read or sleep (e.g., travelling to/from airport, lining up for security, etc) is a good time to brain storm for work.

Lately, I've used fairly long flights as a time to review and tag photos in Lightroom. (In my spare time, I'm a photography student and volunteer photographer for some animal rescue groups, so I have a lot of photos to handle.) I review images, add metadata, name them, etc. It's semi-mindless but really useful to do, and a flight is a good time to do it.

If I absolutely have to do some "day job" work on a plane or lounge, I can fairly passive tasks like read reports, proofread drafts or input data into Excel. But I can't do anything that requires concentrated thought (like drafting a memo).