I don't have a "wow" gift for my 3.5 year old. He wants a bunch of things, but only small items, so I'm not sure what to do there.
On the other hand, my older son (7) wants a lot of "wow" gifts. His big gift will be Lego Hogwarts. I think we'll also get him the Skylander Wii games, so (IMO) that's another "wow" gift.