The Set Up:This one will require you to know what your wife actually likes, which shouldn't be a problem. If however you have no idea what your wife likes, not to worry (unless she finds out), some simple "recon" can give you a clue.
The Planning: First, you can always just come out and ask her (make up a reason), but if you want to embrace your inner James Bond, then check around the house for clues (books/magazines she's reading, her favorite movies, things she DVR's, etc). Unless you've actually spent time with the C.I.A. don't try to do any in depth snooping, she will find out, and she will be pissed.
Don't ask her friends either, no matter how much they promise to keep the secret, they'll talk (they always talk). Keep this on a "need-to-know" basis; remember, surprise counts for half your grade.
This is one where attention to detail will make all the difference; the more "real" you can make it, the more amazed she'll be. Study, study, study is the best advice I can give you for this one, while it will more than likely be easy to find source material, you need to be willing to put in the extra effort.
Example: My wife is a huge fan of the Pirates of the Caribbean movies (aka Johnny Depp), so while she was at work one day, the kids and I turned the house into a pirate paradise. We went all out with the decorations, costumes, music, you name it. We had a pirate style dinner, things like turkey legs, corn on the cob, and lots of stuff to eat with your hands (and I made sure we had plenty of rum for after the kids went to bed).