What gift could I buy for a friend who just got an iPhone?

One of my close friends got an iPhone for Christmas. Her birthday is coming up and she told me she would like something that possibly goes with that. Now, I have no idea what she means because I don’t own one. I also believe that she has no idea what she means because she is terribly un-tech savvy. So is there anything I could get her which would supplement her iPhone?

I already bought her one gift so let’s keep the budget as 25$ max. Worst comes to worse, maybe just an iTunes gift card? Thanks!