Yes, hats! Lots! Generally putting suncream on babies can get fraught, so those all covering swimsuits minimise the amount you both have to go through!!
Are you going long or short haul?
I’d take lots of light and small toys - inflatable beach balls, stacking cups, small cars, balloons (massive winner in our house).
Lots of snacks for the plane, filling non-sugary ones like bananas, that could replace a meal if things go wrong at any point!
Also a really well packed nappy bag for the plane.....spare clothes in a plastic sandwich bag (you can put the ruined ones back in the plastic bag and zip it up if needed). Ditto spare top for you, just in case. Lots of nappies and wipes and bags. Calpol.
Take a comforter or dummy that they can suck on during takeoff and landing to avoid ears being uncomfortable.
We took our pram and were able to walk to the plane steps with it, so so helpful!
Also we took stuff to make bedtime feel more homely - their own flat sheet, little projector, and I even mixed some of our usual fabric softener with water in a spray bottle and sprayed it round the room at bedtime to make it smell like home. That probably sounds like overkill, but I’m certain it helps my small ones when everything is new and overwhelming!!
Good luck, I’m sure you’ll have a blast!