we went on holiday last month with dd1 (3.7, ASD) and dd2 (14 months). We had a 10 hour flight, away for 1 days.
Things we did which seemed to help:
bought dd1 her own suitcase (a trunki one) which she fell in love with (has never liked ride-on stuff before, so god knows why this was ok) and we packed it with her toys/books for the journey. We bought board book versions of some of her favourites as easier to handle in a constricted space (I also made a reduced, laminated version of one that I couldn't get in board book)
took enough food and snacks for the trip (and then some - dds are GF/CF so runningout was not an option). Also managed to take some drinks on board - we got a letter form our doctor stating medical need for the food/drink in case there was a problem at security, but we got through ok.
we flew business class, which worked well for us as lots of space for the dds to stretch out in, but not always an option (thank god for air miles - we upgraded using ours).
we told everyone we came across that dd1 was autistic, and staff were all very happy to help. do call the airline and talk to them; also check airline website, and airport website too.
If your dd watches dvds etc, I'd say a portable dvd is a must.
we booked self-catering, as needed to control food input. we found an all-suites hotel, so had 2 bedrooms and a kitchen & sitting room. this meant we could keep meals as similar to at home as possible (meal times are a big thing for dd1)
we tried as far as possible to keep the general structure of the day as similar to home, and food as close as possible to what she ate at home, and it worked for us. dd1 was a complete star all through the holiday, and even enjoyed some new things.