There may be nothing more happening during halftime than they review the plays, and what worked well, they will continue, what didn't work, they don't do any more that game. And that they review the game plan and let the players know a particular play or two or three that they plan to run, so the players can be prepared a bit more for that, not having to think about the myriad of plays in the playbook that could be run.
But as a practical matter, not a lot you can do in the halftime, first the players have to get off the field and back on the field, in some stadiums it's more than a few seconds' walk. Players also may go for a potty break, get a snack, have ankles re-taped or equipment modified or changed, get minor injuries looked at, and have positional meetings. Any changes have to be based on what went on in the days before the game, just a re-fresher so to speak.
Not going to make huge changes, just no time for that, though minor changes can and do create some big plays....
