This one is simple.  What a woman wears under her cloths makes a difference as to how sexy she feels.  No one, can see her underwear, but she knows its there and she feels like it looks.  So try this:

When she's in the shower, get a pencil and paper.  Go into her dresser drawer and pull out three pairs of panties.  I say three because sometimes there will be different sizes depending on cut.  If she likes thongs, then look for those (many women don't because they "ride up" too much).  In any case, write down the sizes (some may be a number and some a S / M / L / XL).  Do the same for bras. 

Now - next time you are out by yourself - on the way home or something, stop by a Victoria's Secret - or shop on line.  Ask for one of the girls to help you pick some things out for your wife (don't be ashamed - trust me: they'll think you're quite a man for what you're doing for your wife).  Get 5 or 6 sexy comfortable panties and 1 or 2 bras.  I say 1 or 2 bras because they are expensive and women tend to be very picky about they're bras.

Then have them gift boxed with a little note that says something like "For you, my love.  Just because..."

Leave it out for her on the bed (that you just made) before you go to work.  If you don't get a call from a very happy woman, I'd be surprised (of course, don't be upset if she doesn't call - this is for HER to feel more sexy - not for you to get accolades - be in it for the long term).