We only have a 3+hr trip to WDW so we don't drag the DVD player w/ us at all.
I've flown w/ dd at 17mo and 28mo and ds at 24mo...this is what we generally pack:
Travel Size Magnadoodle (they call them Doodle Pros now) - this takes the place of crayons, markers, crayola wonder markers and aquadoodles. All of the latter have pieces that you WILL be diving for on the floor (and the refilling of the aquadoodle). The Magnadoodle is all kinds of compact and the "pen" is attached on the string. Dd started playing w/ hers at 14mo!
Character Sets from TDS - The hits w/ ds were the Mickey Mouse Club set and Cars (either of the 2 available sets). You can pick them up 2/$20 on sale generally.... PRICELESS!
Board Books - Pack two. Ds had us read them over and over!
Sticker Books - Something small, and I found a few good Disney themed ones at the Dollar Store and independent toy stores.
Colorforms - They are great to stick to the airplane window!
Most of the above was "new" and saved for the trip. We also packed some standbys, including ds' favorite Thomas the Tank Engine trains...he is obsessed w/ them!
I picked up a small backpack at Target (Thomas themed...go figure!) that held all of ds' toys (when dd was younger we used the bonus backpack we got from the Dr. Suess book club). This way all the airplane amusements are totally separate from the diaper bag and mom's/dad's stuff.
When we traveled in December w/ ds (2yo) and dd(6yo) it worked out well. Dd carried her own backpack of stuff, dh had his, I had the diaper bag backpack and ds' backpack went under the stroller. Dh pulled the collapsible travel cart w/ ds' car seat on it. On the plane, everything goes in the overhead bins except the kids' two backpacks!
HTH!