I'm thinking about going bagless this year (bad shoulders).  I will still carry some headache remedy.  One year at Origins, I developed a splitting headache; I'll never forget the kind strangers who heard my plight and gave me some Tylenol.  

As a 35 year con veteran as well as an Army veteran, there is one thing that's always in my bag- a spare pair of socks.  Trust me, the energizing feeling you get when you change your socks mid-day is amazing.  And the older I've gotten, the more I absolutely rely on that swap.  Do yourself a favor and bring a spare pair, and when your dogs start barking, swap them out and then thank me later.
 

My old satchel was a Bag of Holding from Thinkgeek that had served me well for around 7 years, but it was finally time to retire it. Lived up to its namesake.

However, with a smaller satchel on hand, and due to not shopping much at the con (I volunteer at a booth and enjoy that experience plenty), my usual kit is as follows;

- Battery pack and charging cables (ideally for iphone and android, so I can help out those around me)
- lozenges (talking almost non-stop for hours on end takes a toll)
- dice set (whether I get to play in other games or not, I'm always prepared to roll for initiative)
- challenge coins (my own, plus a couple of spares in case I come across someone who shares the same fandom)
- booth odds and ends (a few band-aids, kleenex, tylenol/advil, small water bottle, small snack, pins, things that might be used during setup, teardown, or useful while on duty, especially if they can be replenished at the booth).
- a spare shirt or light hoody is common in the kit, but not an absolute. Indianapolis is warm/dry enough that I don't consider it a necessity, but if I'm on the go, traveling, or at a con in rainier/cooler climates, it's nice to have on hand. The former can even be good at Indianapolis, nobody is at their freshest after running games for most of the day, and sometimes we just don't have time or feel like the trek back to the hotel before heading off to dinner.

- This year I'll probably add a small deck box and sleeves in the hopes of sneaking in a pre-release event for the new Magic set coming out that weekend, but I'll probably leave my built decks at home or the hotel. Even the least expensive of them is more than I'd want to be worrying about all day.

That's off the cuff, I'm sure there are other things I'm overlooking, but I try to balance traveling fairly light while also being prepared for common contingencies, and a few outside the box ones while I'm at it.