we did this 18 months ago with dd1 then aged 4 and dd2 then aged 2. dd1 also has severe ASD.
ipod touch/ipad (depending on how far your budget can stretch!) is invaluable - much better battery life than a portable dvd player, much smaller/lighter too.
you can watch films/programmes on it, listen to favourite music and also the apps are amazing - everything form drawing/colouring to puzzles, simple letter/number games, even picture books.
kept my two quiet for ages.
sticker books are a must. you can also get little magnetic board playscenes - we have a hungry caterpillar one and a Night Garden one (prob both form ELC). big hit last holiday was also a fabric book of the Hngry caterpillar, with washable pens - so dc colour in the book, then you bung it in the wash and they do it all over again. marvellous.
the aquadraw mats are good too -endless colouring with no mess.
one thing we did was to take essential nighttime comforters on board with us - so come bedtime, we changed the girls into pyjamas, had their bedtime cuddlies, read the right stories etc. it really helped settle them down, and they both slept 7/8 hours, which was really good.