J.C.
Are you staying at a hotel? If so, ask for a crib and sheet. They should have them for you. You can rent a crib from rental services in the area as well if you are staying with friends. No need to purchase a pack and play - which are super heavy, by the way, and you'd have to pay the airline to travel with it.
If you are buying him a seat, just bring the car seat on board. You can pop it on top of a rolling suitcase or the stroller. Also, I assume that you are bringing a stroller? You can keep it until you board and then they will have it put in the cargo hold for you for free. Just call the airline and confirm.
Denver - FUN!! Be sure to go to Cherry Creek.