There are musical instrument kits out there somewhere that have all the instruments you may remember from elementary school packaged in a hat-box-shaped box that doubles as a drum.
I raised three children and sometimes they enjoy the box the toy the came in more than the actual toy.
That being said, I think an empty box would go over like a lead balloon.
Books never failed for us as an occasional surprise gift.
I agree with Kevbo that something that makes noise is always a safe bet with a two year old.
Books are great. Especially ones they can relate to. Such as being a big sister/brother. Elmo has a book about potty training. Or you can find some really good books about the things she is into. Like Dora, Elmo, princesses. It encourages them at a young age to read too.