Here is my list of ideas:

Average ideas . Get her a voucher for her fave shop
. CDS?
. DVD?
. Clothes
. A photo album of ur twos most special memories
. Jewellery
. Nail varnish an make up set
. Scary movie nightt
. Take her out for a special girls day to the cinema
. Massage chair cushion
. Stuff for bath
.Retro Webcam?
.A cheapish digital cameraa
. Box set of her fave film
.Book a hall or summit for suprise disco
. Bake her a cake YOURSELF LOL

Special ideas . A Photoshoot of you her and a few other friends ive always wanted to do that!
. Tickets to a theme park?
. A Choclate fountain!!
. A Coctail fountain!! Though as shes turning 13 you might wanna get a fruit one
. Book a meal and invite close friends
. A suprise Confetti Cannon!
. A suprise display of fireworks
. Get her a suprise make over
.Boogie bus hummer or limo (Get everyone to put money towards)
.Dress up to the concert buy her a outfit to dress up in like ravers or punks LOL

All i have so far sorryy
hope this helps you think of some ideas above and beyondd hehe!

A really cute frame with a picture of the two of you in it.
A memory journal? If she likes to scrap book- then some really cute stuff from Michaels. Michaels has alot of great priced little things- so you could get a little bag, or a little cute box , and fill it with little goodies.
Cute notebooks, journals,pens,candles,favorite candy,stuffed animal etc…Things that you know she likes and would be fun.
Hope that helps.