No gift will provoke as strong an emotional reaction than a gift that is personal and specific to the bond you share. Every couple has inside jokes, landmark moments in their relationship, and first times. Buy a present that touches on one of these things, and you've struck gift-giving gold.
A mystery gift certificate for a night out that starts at the place you had your first kiss, and ends at the place you had your first date? Win. A photo album of your vacation together? A small painting of a hot air balloon because when you first confessed your feelings for her you said, "I always feel so high around you"? Jackpot.
Clearly, these are hypothetical, wildly specific situations. But, take a look at your relationship and think up creative ways to make moments tangible.