Understand the concern as I travel to SF with my 2.5 yo in tow for a few times already. It was a bit of a challenge but looking at it positively, it was quite an experience! :-)
These were what I brought on hand carry:
- 4 small pouches--each containing small toys and/ or small board game sets(to take out one pouch at one time so that the freshness of playing with the toys remains; also choose toys that does not produce too much noise that disturbs other passengers).
- a few small books
- laptop/ipad with cartoons and games (I find that the cartoon movies/ shows onboard not that suitable for 2.5 yo but 5 yo may be alright)
- washable crayons and markers (in ziploc bags) with cartoon characters activity booklets
- lots of her favourite snacks (best to be low in sugar content as young children may get too hyper and not good for long travels).
- bring 2 sets of change of clothes (for spills and leaks), small towel & wet wipes.
Think depending on airlines, the air stewardess also gives some activity packs for children. So this comes a plus and addition to what you brought.
Generally when going or SF, the first lap of the journey (either to Tokyo, Hong Kong, Taipei, or Seoul) is easier since it'll take 4-6 hours. So I find that using the activity packs from the airline is adequate. So I tend to ration the activities so that on the last few hours of the 2nd lap (with 10-15hours) still hanging, I can still provide activities and toys to a "cranky" kid...
I also find that during the transit time (though may be short), it's good for the children to walk around instead of sitting at the waiting area so that they can spend their energy. Since baby, my dear daughter sleeps lesser than her counterparts. On my first trip to SF, I wasn't sure if she can sleep so I asked her pediatrician to give medicine like for allergies which makes her sleepy. Thank God I didn't need to use it. That's a little drastic but good to just prepare and use as last resort...
Also onboard, hydrating is very necessary for children. They get less cranky if well hydrated. For their sleeping time, try to stretch their naps or sleep to be nearer to SF time so that they can adjust to SF time faster.
I'm not sure if your 2.5yo is potty trained but if not, may need to use better diapers (for nite use kind) so that it'll not leak or need to change so many times. Changing diapers in the toilet can be quite a hassle. So pull-up pants type is also good.
Hope this helps.