Does she have a favorite flower? Roses are really overused. If she isn't particular on the kind of flower, buy exotics or something like calla lilies. Or, how about a gourmet basket with wine or chocolate?
If she has a favorite perfume, you can buy the accessories that accompany it (i.e., lotion, powder, etc.).
Does she collect anything? Pay attention to what she likes (i.e., if you are shopping and she says, "Oh, I love that pair of earrings/purse," then make a mental note).
Cook a nice dinner for her, or take her out to a new restaurant. Or, take her to the first restaurant you ever went to (if available).
Cards (you have to write more than three words, guys) are always very nice, esp. when unexpected. When my husband went out of town last year for about four days, he hid cards in odd places (one for each day) and called me to tell me where to look. It was very nice; of course, he had sweet words in all of them.
Women love personalized items! See below (stuff like that).
Candles, body lotion, etc.
Note: Itís really doing small things consistently.