My primary client is Keller Williams Realty, and their annual convention, called the Keller Williams Family Reunion, happens every February. This year I'll attend my 16th, and I've got my "conference essentials kit" down pat:

It's always a good idea to bring lots of business cards, but hopefully yours, if they have a picture of you at all, have a more recent photo than one taken 11 years ago. (I just got new photos done a couple weeks ago, but I'll share more on that later.)
I don't understand why, but folks find all kinds of reasons to justify not investing $10-25 on a good portable phone battery charger. It's a great tool to have, especially if you're also taking notes and photos of presentation slides on your phone.
The outlet splitter could make you the most popular person in a breakout session room -- I know from first-hand experience.
Most convention centers have big community water bottles, and flimsy plastic cups. Don't bother with them. Bring your own empty water bottle and fill up as you need to. Just test beforehand to make sure the bottle you bring has a tight seal. (Ask me how I know...)

I don't leave for another couple of days, so tell me -- am I missing anything? ;)