The standard gift: A candle.
Impress her with: A Jo Malone personalized candle or perfume. Gather any shred of information from your boyfriend about her preferences (favorite flowers, time of year, vacation spots, colors, etc.), then log on to jomalone.com and chat live with one of their stylists. They'll point you in the right direction after asking a few questions.
What you say when you present it: "Bob said you love vacationing on Martha's Vineyard. I found a candle that will hopefully bring back some memories, with hints of sea kelp and aged beechwood."