the youngest my daughter was when we took her was 22months and the best thing i did was to pay for her own seat. She was given her own seat with an insert which also had a 5 piece harness when she was strapped in she knew she had to stay put. she just enjoyed pulling the window blind up and down. Also took all of the little toys mentioned above and also lots of her favourite story books. I would also buy some juice after going through security as there is not a lot on offer if they don't do fizzy juice. It' usually water but that can get a bit boring.
On our return flight she was getting very tired but likes a cup of milk before sleeping. The flight attendant filled her cup up and just said come back for more if you need it. Never really thought of asking for milk before as just thought they only had the long life little things for the tea.