My favorite accessory is the BudCozy. It’s a fabric cover for earbuds that keeps away thetangles and looks great too.
Battery for iphone:
This is the one I have:
It has 2 AA batteries that can be swapped out. I got mine in japan. But I think you can get it in costco.
Here’s a bunch of power-related accessories:
His and hers iPhone charger:
Another external battery:
And the Navigon iPhone app has is a better GPS than the Garmin i’ve used a number of times.