First off, welcome back to Azeroth! There are some great suggestions in here so far. While it's not necessarily preparation for the expansion, I definitely suggest you check out content from some of the major patches you may have missed out on. We even have pages for each one of them detailing what they added to the game.

Rise of the Zandalari
http://us.battle.net/wow/en/game/patch-notes/4-1

Rage of the Firelands
http://us.battle.net/wow/en/game/patch-notes/4-2

Hour of Twilight
http://us.battle.net/wow/en/game/patch-notes/4-3

Really though, the best thing is to jump right in game and check things out for yourself. Once you get back in, you’ll find plenty to do. Good luck!