Just tell her that you're thinking of buying a bike for someone. I don't know. Perhaps your nice or daughter or some girl. Tell her that you might even get one for yourself. It doesn't matter, as long as you're able to get her inside of the bike shop. Just act like the last thing on your mind is to get her a bike! Once you get her inside the shop, tell her that you want to ride around the block with her. Make it sound and look like something new and romantic, she'll like that! Make sure you stay in the bike shop long enough to get some idea about what type of bike she'll most likely appreciate. Then you'll each pick out two bikes on which to ride. You'll leave one of your credit cards and take off!
Of course, you would have called the LBS ahead of time and made all of the arrangements.
- Slim