I just did a trip cross country with my 2 year old by myself- here's how I (barely) survived:
LOTS of snacks!!!! We probably spent most of the time just eating. Also lots of books- paperback, not board books- too heavy. I bought some little toys that were just for the trip so that they would be more exciting- a Magnadoodle, a magnetic zoo scene where she could take the animals on and off, little animal figurines of her favorite animals, new crayons and a new notebook to draw in, and a new pack of hundreds of stickers which she spent a lot of time sticking all over me and all over the seats.
She is a little older than 20 months, but maybe some of those things will work for you. We also spent a lot of time walking up and down the aisle. Glad you'll have your husband there to help- that should make it better!
good luck!