My wife and I took our then 8 month old son, along with our DD, 7 years, last June. We had a GREAT time; so much so, we went back again in August, LOL.
With that said, he was able to ride a LOT of stuff. Pretty much everything except of course coasters, and wilder rides. Some examples of what he rode, and enjoyed were the jungle cruise, pirates, haunted mansion, small world, peter pan, pretty much everything in fantasyland really, and a host of other things at MK. Epcot was great too, riding Nemo, spaceship earth, the land etc. etc. He also really enjoyed Animal Kingdom, and things like the safair, rafiki's planet watch, and even the nemo show their.
So yeah, there are a LOT of things for a young one to enjoy at WDW. Many more than I have mentioned of course, and the memories you will create will absolutely last a lifetime!
Oh, on a side note. My wife uses a baby ring sling. She used it the entire 11 days we were there last trip, and absolutely swears by it. (Used on both trips actually). It allowed our son to sleep when needed, and still stay "hand held" at all times, even when on the rides. My wife said it was very, very pleasant to be able to hold him, without the arm\back strain. Highly recommended over say the bjorn type, which frankly, are a pain in the back, and don't allow the child to easily switch from facing you, to facing a away from you.