Every ride in Fantasyland at MK. Wedway in Tomarrowland... the jungle cruise, Pirates, Train around MK, Spaceship Earth @Epcot...
I really could go on... there really is alot that you can go on with your 20 month old... we did that as well when our daughter was almost 2. That is the beauty of Disney, the whole family really can get alot out of it. The obvious ones like, ToT, Spash, Big Thunder, etc, have this wonderful thing called baby swap... everyone can ride. For example your DH and 5 yo can ride Barnstormer while you wait with the baby, when they get off you and your 5 yo can use the pass that you Dh was given to ride yourself while he waits with the baby. You won't be as "handicapped" at Disney as you would be at some other theme park.
Congrats on your trip... your in for a wonderful time!!