We’ve recently completed two short trips, one by plane and one by train. On both occasions, The Boy had his own small backpack to carry. He was able to choose a few small toys and activities to take along. This worked quite well and freed up a bit of room in our own carry-ons. The best activity we had on both trips was our little baggie of stickers (cut from old address labels) and a few sheets of blank paper. The Boy was quite well occupied pulling the stickers off their backing and ‘decorating’ the paper with them. This activity was cheap (free actually) and very light-weight.