Thanks for all the suggestions. Here is a little more info:
-FIL recently passed away, they were married 42 years. They went camping a lot, but that was pretty much it. She's never really "vacationed" like this. And she is not one to do something like this on her own. I couldn't imagine her hanging out at DTD or WS by herself.
-Coming along was her idea. We were watching the Christmas parade while opening gifts and she said, "I'd love to go to WDW one day." Well, actually she said Disneyland, but then clarified by saying "Or whichever one you guys go to." She then brought it up going with us again a few weeks ago when we mentioned planning our next trip. So, she kind of invited herself, LOL.
-She knows ZERO about Disney. If she sees a picture of the castle, she knows that's a Disney park, but that's about it. She knows we have lots of fun because we talk about it all the time, which is why I think she wants to go. Since FIL's passing, she spends a lot of time with us and our kids.
-We will most likely get a condo so there is plenty of room. I like my MIL and all, but I don't want to be crammed in a hotel room with her for a week.