Anything without a height restriction you choose to take him on. Which means that if you want to take him into Pirates of the Carribean or Haunted Mansion - you can (and at 15 months he will probably be far too little to comprehend that these are scary rides for four and five year olds).
At 15 months he will especially like: Small World, Peter Pan's Flight, Pooh, the Tiki Birds, Country Bear Jamboree at all MK.
People watching at Epcot (I think the only rides with a height restriction there are test track and body wars). Looking at the fish at The Living Seas.
Shows at MGM (although the 3D movies are hard until the kids are 3 or 4 - they don't leave their glasses on).
The zoo portions of AK and the safari.
You know about Baby Swap? Both parents go through the line for something with a height restriction, one rides while the other holds the baby and as soon as Dad gets off, Mom hands him the baby and takes her turn.