Some stuff that we used for my VERY active baby last time we flew (He was 12ish mos, I think).
take little things that can occupy their hands. They can't use their gross motor skills enough on the plane, so bring things that will use their fine motor skills. Some ideas: a thing of floss that he can unravel, crayons and paper (even if it's just you writing, it might entertain him for a few minutes), a pen that has a clickable end- let him click the end, a box of tissues can provide lots of entertainment- they are like magic!! Maybe a ziplock baggie to put toys in and out of. Oh! I brought some of those flavored gerber puffs (the only time I've bought them), and my ds had a blast reaching in the container to get them out.
Also, you can bring stuff that makes noise, as long as it's not super loud. It's so loud on planes, that other people probably won't even be able to hear it. We brought a kids cell phone, and even *we* couldn't hear it unless it was right up to our ears. And if it comes down to it, I think people would rather hear a toy than a crying kid.
What else- depending on how active your kid is (like I said, mine was really really active), you might find that trips to the bathroom are necessary to keep your sanity. For me, it was seriously the only place I could relax. lol.
Becky, partner to Teague, SAHM to Keagan (7yo), Jonah (2yo)