Take her on Big Thunder Mountain first, if she wants to go; it's the tamest coaster of the
Disneyland mountain range. If she enjoys it, the Matterhorn is next (if it's open), and then Space Mountain.
You might also try Star Tours. It's a simulator, not a coaster, but it does make you think you're going through coasterlike motions (dips, bumps, and high speeds). It's another one with neck-and-back warnings, though. Oh yeah, note on Star Tours: The further back and to the side you sit, the greater the effects will be. So if you sit front and center, you'll have a fairly mild ride, while if you sit in the last row on the edge, it'll be pretty bumpy.
While I'm on the subject, here's a Star Tours fun fact: The little G-2 droids who talk to you in the queue didn't start their lives as droids. They used to be barbershop geese from the old America Sings attraction, and the AudioAnimatronics got ported over when Star Tours opened. Check out their feet - they still look like goose feet!