No fuel belt, no music for me. I've found that Disney has a lot of well-stocked water stops, and I don't bring music because I want to fully soak in everything that is going on around me.
Other things to bring in addition to what you said (Garmin, sunglasses, fuel - which I keep in my running skirt pockets, so no fuel belt needed):
- Magic Band/park ticket...and I think I've brought my ID in the past
- Phone (only race course where I find myself pulling off to the side to text people that are trying to find me)
- CAMERA (only race course where I take pictures)!
- Cash
- Kleenex/TP...trust me

- Extra hair tie
- Throw-away clothes for waiting at the start (haven't needed them for the past few years but you never know)
- I also bring my breakfast with me to eat while waiting in the start area (coffee, bagel with peanut butter)
- Apply sunscreen beforehand
I think that's it. I'll have to think about this more.