Jorina once gave me this really useful list of what to pack when going on a holiday/plane with your toddler. A lot of it makes sense, and I'm sure she can point you to where she got that list if you write her.
But here're some of mine to consider:
Stock taking is serious business
After a long debate with your worldly-wise toddler, you have finally decided on the toys and the books that you will bring on the trip to keep both parties sane. When you travel on the road (i.e. from hotel to hotel), however, there is a chance you might forget what you've packed. So a toy or a book might get kicked (or thrown) by your toddler under the hotel bed/cupboard/blanket, and you won't miss it until you unpack at the next hotel after a gruelling four-hour car ride.
Your nerves are frazzled by then, and the last thing you want to hear is a toddler screaming: "THOMAAAASSSS!!!"
These toys and videos kept him out of my hair
To help you keep stock on the inventory of toys and books you've packed, take a picture of ALL the toys and books you've packed with your digital camera. It will help you remember if you've brought, for example, five railway tracks, eight books, six Thomas and Friends tank engines.
Whip out that picture to check you have exactly those items in your luggage before you lock it and throw it into the boot of the car.
Minimise the bulk
Don't bring a toy truck the size of your pet cat. And don't bring any hardcover books.
One of the tips in Jorina's list is to bring one set of extra clothing for you AND the toddler/baby. Reason is clear: Accidents/Spillage do happen.
I did that the first trip, however, it really added to the bulk and weight of the hand carry luggage. I got lazy, but learnt my lesson on my third trip with Kai to Australia. (Think orange juice. Mess.)
A good compromise is to bring an additional set for your toddler (their clothes are tiny=not bulky anyway). Bring just another top for yourself and pray the gravy chicken doesn't land on your pants.
A good squeeze can land you in trouble
Airlines sometimes serve juices in kid-sized tetra packs. Give that to an excited toddler or a toddler who's learning how to drink through straws, and you're heading for potential disaster. Same goes with a plastic cup filled with juice.
Kai has squeezed juice out through straws and spilled juice from cups. A better idea, if your toddler lets you, is to squeeze/pour the juice into his sippy cup. This means: Bring your sippy cup on board.