Buy lots of small 'pocket money' type toys (if you have a John Lewis near you they have a pocket money section in their toy department which contains all sorts of odds and ends. Present them to your toddler for the first time whilst mild flight - put them in a big bag so that he can rummage around for them and enjoy discovering them.
Bob the Builder toys from Woolies - the small ones - 5 for £10. DS (2.4) had them for months before but loved them on the flight. Small enough not to get lost but easy enough to carry. Books. DH taped Kipper and played it through his lap top, DS listened to that for ages.
We also bought him a small suitcase on wheels (a penguin!) and that was great in the airport to slow him down. He looked rather cute too
We took our 2 year old to the States in December -bought lots of pocket money toys and wrapped them up and then let her open them at strategic moments! Sticker books and the little etch a sketch from ELC were popular as was a colouring in book and pencils. We also bought a portable DVD player (cheapie one) and took a Fimbles and Balamory DVD for her to watch. I think fuzzy felt might also be a winner as long as you don't mind losing some of it! Good luck!
I found Play Doh very good on long-haul flights - just those little party tubs. Younger toddlers just squish it, and it probably makes a dreadful mess on the carpet (!), but it kept mine quiet for quite a time. Also sticker books etc. Most long flights have at seat video too, so they can watch the children's selection over and over.
We took our dd at about 18 months on a long haul and I bought lots of little glittery boxes with lids from the card shop and filled them with inexpensive little bits and bobs. Twinkley art and crafy pom poms, mini animals, pipe cleaners, just bits she could fiddle with. Gave her different things at different times, alternated it with food, then a walk up and down, then a nap, then back to the bits. Stickers worked well and fuzzy felt. If you have loads of inexpensive bits it doesn't matter if you loose any. Little puzzle books and reading books. A hard lolly on a stick can last for about 1/2 hour (not normally given, but good in times of desperate measures) Take their own bag packed with bits and bobs and don't get it all out at once otherwise it gets chaotic on the small lap tray. Pack everything away before you get out the next thing. Good luck, just use a bit of imagination and visit a pound shop for nic nacks.