Depends on where I would be staying and what time we arrived. Being a bit of a lardy I don't sleep on planes so I'm usually not too lively by the time I clear customs,get bags, drive to accommodation etc.
If I was off site, then just grab something to eat, maybe grocery shop ( if in a villa) and unpack (maybe jump in the pool).
If onsite, and it was fairly early, depending resort either take a trip to DTD or Boardwalk soak up a little atmosphere get something to eat and trot off to bed. I've done stuff like take the kids swimming while my ex unpacked (she prefered to be able to do it without being interupted by the kids), go to the supermarket to stock up on supplies/brekkie/drinks .
Generally I didn't plan anything, just get there and see how we all felt. Trouble with trying to do anything is, by the time you factor in checking in, unpacking, travelling to wherever, doing something for even just an hour, travelling back you're going to be looking at 9.30-10.00 ( or 3.00 AM UK TIME) at the earliest. I'd much rather get to bed early and wake up the next morning feeling ready to take on the parks without any of the group looking/feeling/behaving like they needed a few more hours of sleep.