I would say don't stress yourself out about what to carry. We've traveled a bit (with 3 1/2 year old twins) and I found I just got exasperated because I spent all this time packing the toys and they didn't even want them.
What we brought that has been most successful:
Magna Doodle (or something similar) - I do not recommend crayons for the simple fact that you will spend 5 hours bending down to pick them up. (I personally don't find that fun).
We have however brought a pencil and little notepads and have had a little more success with them holding on to it.
usually we bring one or two storybooks.
And if they have little character toys/figurines, I bring one each so they can have pretend time on the plane. That can last a long while.
That's all I bring.
As far as snacks, I usually bring a couple packs of fruit snacks. Depending which airline you might get some pretzels, but you wouldn't give your child more than 2 snacks in 5 hours, so don't worry about packing the cupboard.
Airlines will generally let you bring in a sippy cup of water, they've recommended at the airport security that I carry my girls through security so it looks like they are smaller than they are and need their sippy cup. I've never had one taken away or emptied yet.
It's really about your style. I have one friend that wrapped a small toy for each 30 minutes on the plane. I couldn't even imagine doing that.
I would sit on the isle with the two kids on the inside (so they don't feel like they can get up and walk in the isle). Make friends with the flight attendants, that goes a long way if the kids decide to be super cranky!