Double check the size requirements before purchasing a CARES harness. My almost 3 year olds are still too small for it, so we always bring the car seats. If she is big enough for one, you can rent them through people on eBay.
For next month's trip, I'm making bento-type snack boxes to give them some variety. I always bring their straw cups so they can drink during take off and landing to open their ears. Color wonder markers, band aids to play with (stick it on mommy, stick it on the seat, stick it on dolly; comes off a lot easier than real stickers), iPad for a last resort. In the past I took cheerios and put them on a string like a candy necklace and they loved that. Check the dollar spot at Target, they always have Disney-themed small toys.