Hi Kennon!
Yes it is possible AND enjoyable with an 18 month old! I've taken my kids when they were 7 months old, 9 months old and 24 months. It is a little harder--because naps, meals and bedtimes are a must. But you can still see and do so much PLUS the little guys actually love it!
There is lots your 18 month old can do as far as attractions and shows. Check out the
Traveling With Little Ones page to see age appropriate attractions and other great tips.
My big tip is to find and use the Baby Care Centers! There is one at every park and has a tv and books for kids, (semi) private nursing rooms, changing tables, high chairs, microwaves and more. Such a good little air-conditioned stop for you and your little one.
My other tip is to be prepared--have the favorite, toy, blanket, snack, etc. with you. Also hide away some new stuff they've never seen to surprise them when you need it. If they sleep well in the stroller, make sure you bring it with you.
And don't be afraid to leave the parks for a naps--your 13 year old can enjoy the resort while your baby sleeps. And then you can head out again at the end of the day! :)