E.S.
We just got back from a week in Vegas with my 2 year old. We were staying in condos off the strip, but went to the strip plenty of times. My son had a good time at the habitat in the Flamingo hotel. They had lots of ducks, fish and flamingoes for him to look at. He also liked the waterfalls outside the Wynn hotel.
As others suggested, I would bring a good stroller because just walking up/down the strip and looking at the various people dressed up as characters was a great time for him. We even took pictures with the Elmo and Cookie Monster characters.
Mandalay Bay has an aquarium that is pretty good if they like looking at fish. If you decide to go there, I would suggest going to the half price ticket booth (tix4tonight.com) place for discounted tickets.
If you have a car, we found an outside playground at the Town Square Mall which is a few miles south of Mandalay Bay. My son had such a good time playing there that we went back a couple times. They also have great shopping in a more quiet setting.
I had read before we went that Circus, Circus does shows throughout the day, but we didn't make it to that.
As for traveling, we made sure to give him something to drink as we were taking off/landing to help with his ears. Bring a portable DVD player or an IPAD/IPOD for them to play with. I also had "paint with water" books with me because it was new to him so he was enthralled with it and yet, it didn't make a mess.
Good luck and have a great time!