The magazines provided by the airline are usually full of pictures. Look at them with your child and challenge him to find a particular thing on each page. Challenge an older toddler to search the whole magazine himself to find the object you have selected. You can even go further and ask for a type of thing, a striped animal for example, and have him find something in that category.
You can keep your child entertained with some simple guessing games that can be played from the seat of the aircraft. Use three plastic cups and hide something under one of them. Move the cups around and get him to guess which cup the object is under. Blindfold him with a scarf and give him various objects to feel and guess what they are.
A great game for encouraging your child to think about the journey is map making. Take a few sheets of paper with you and draw a simple map of the journey you are taking. Explain to him where you are going to and from and show him where various people he knows live on the map. Next, get him to imagine what it would be like to take the journey over land and imagine the adventures that he would have at each point along the way. Let him draw the adventures on to the map.
Flying is a pretty amazing experience for toddlers, if only for a short while. Try and engage them with the experience. If you are flying over land take them to the window and play a simple version of eye spy. Rather than choosing a letter, simply name things you can see from the window and see if he can see it too. Take turns to let him pick something and make a show out of trying to find it.
