Our boy will be 3 in September, and we've traveled with him on an airplane about 2 times every year for those 3 years of his life (seeing grandparents). I found that the best way for us was to put away age-appropriate toys for awhile before that would work well and keep him entertained for awhile - I didn't bring toys that he played with everyday before the trip. You might even want to buy a "new" (second-hand store or garage sale is more economical) multi-function quiet toy. Then, of course, I found little compact things around the house like straws, small kitchen gadgets, or anything that was compact and I thought would entertain him for a decent amount of time (books?). Then, of course, I brought food that was slow to eat/drink that would take time too. All these things I did not use until I had to, so if he was interested for a little while looking out the window or playing with the safety card, then I'd let him do that instead. At some age, I remember he really enjoyed playing with the plastic cup they give you for beverages - I just asked for one. Finally, I always kept in mind that I may have to get up and walk with him down the aisle at some point if he got too agitated.
Hope that helps!