I've taken my now fiancee up a bunch of times. On our second date, we took a 182 down to Monterey, CA and spent the day there. She's been hooked ever since. Since then we've done everything from flying a Baron across the USA to putzing down to Baja California in a 172. She's now working on her private multi.
Make it something scenic and romantic. Ask if she wants to fly the plane, kinda show her how to bank, climb, descend, etc. Fly over her house if you can. And make sure to do it all around sunset....
And don't forget to post the pics on here after!!