What a great Valentine's present, you're a lucky man!
The suggestions already made by DRT and KillerB are all very sound. Graham's is a very nice one to visit, because they have a bar where you can order the special stuff (at an extra price of course, but still very much worth it). I don't believe the Kopke lodge is open to visitors anymore, but they do have a bar on the quay where you can taste their ports. A good replacement for the Kopke lodge would be Krohn. Here you really get the feel of a small, working lodge. They make some very fine colheitas as well.
If you want to give a nice present to your wife in return, take her for lunch to the restaurant in the Taylor's lodge. The food is good, the view excellent and there are some very good wines and ports on the list.
Finally, a visit to Porto is incomplete without a visit to Vinologia. Make sure you drop by there and tell them the guys at TPF sent you.