Novelty is important! Other moms have suggested most of the things I do (new books, inflatable beach ball, aquadoodle), but the key thing I'd suggest for you is that some of the "toys" be new or novel (something he has never played with before).
With books, to take some of the weight and space away, and allow space to bring a few more books, we go with paperback type books. Most book stores carry them, and you can order them from Scholastic Books (online). They are just like the hardcover books, but cost less, and are very light weight for traveling. Little hands can damage those books more easily, but we taught our son to be gentle with his, "soft books." Now he's older, and when we travel he picks the "soft books" to bring along.
He LOVES Aquadoodle.
We also bring along soft squishy balls. They are nice fidgets for him on the plane, and when you thrown them around inside, they are less likely to spot walls or break things.
A favorite "lovie" is always a good thing, but be careful! Those are easy to leave behind or to fall out of a bag while crossing the airport. We try and give our son a new huggable toy just as we're leaving. We go with new because if he does love it, and we do lose it, we can usually find an identical replacement! Harder to do with a well-loved toy!
Good luck!