Here's some other toy ideas beyond the DVD idea (highly recommended if your kid is into any shows) that work on my child:

*Sticker books (Costco sells big thick ones for less than $10).
*Magnadoodle (mine is the one with a lot of parts, which is kinda tough on a plane, but she loves it).
*Try some of the very basic "workbooks" available at Target, etc. You have to help them through it, but mine loved the challenge. Coloring/drawing is ok but usually isn't that interesting and involves using every single marker/crayon.
*Comfort items from home (blankie, favorite bed-time toy). Mine loves having them handy at times even if she isn't going to sleep.
*A few new books to read if yours likes story time.
*A few new small toys (like the stuff handed out in happy meals). New is good; time to assemble is entertaining; and if it gets lost in the seats it isn't the tragedy of losing the favorite toy.
*Food. Pack some treats to supplement the plane food (if any). Toddlers take forever to eat, and would you rather have a happy kid, or hold the line on junk food?
*Get up and walk up and down the aisle when the food/beverage carts aren't out.

I don't agree with the split-em-up approach; usually my spouse and I alternate--one of us is in the seat adjacent to the toddler, and has the full-time job of keeping the little one entertained. The other parent is "support" and is responsible for getting all the junk in and out of the bags for the entertaining parent. Nobody gets to "relax and enjoy the flight"--our job is to keep our little one entertained so others have a fighting chance...

Good luck!