Hmm. Some of the Tiffany silver accessories are very nice and relatively inexpensive... how about a "Return to Tiffany" keychain?
Could she use a PalmPilot? I swore by my electronic organizer when I was in high school... PalmPilots hadn't come out yet... if she's like me, she'll need a way to keep track of everyone's phone numbers
Or, if her dad is agreeable, you could take her to a beauty salon for a facial or a makeover (just go light on the makeup, a 13-year-old doesn't need a lot).
It all depends on what her dad agrees to and what you're willing to spend.