I have two teenage girls and a limited budget so I have plenty of experience trying to come up with good parties.
On my daughter's 16th, when the kids would arrive, I took a picture of her with each kid. I then had the kid write something they liked about her or a memory...some sort of note for her to remember on a piece of cardstock. I then made a scrapbook of each person and other pages of the party itself.
When my kids get a certain age, they get harder to buy for and really prefer money. I hate handing over money, it seems impersonal somehow. So one time we went and got her birthday money in one's and hid them around the house.
She unwrapped a one dollar bill that had a clue as to where the next dollar bill would be. Her and her friends had a blast looking for each dollar and watching the pile grow. (It seems like more when the are ones). Or you can also tape the ones end to end to make a dollar "Cake".
I also got one of my daughters a build-a-bear which might seem kinda young for someone her age but it turned out ok. She happens to be a...not so much emo, but one that dresses differently. Her bear had skinny jeans, converse and hot pink w/ black polka dots shirt, and an ipod sticking out of its head just like her. It was her mini-me.
We also have a tradition we do at their parties. We go to the dollar store and get glow sticks for all the kids. That night, they will throw them all in the tree. The whole tree lights up and they all look forward to it each year. We have glow sticks up there from years ago.
On a 13th party, we did a snow ball fight in summer. Snow being pantie hose and flour. You cut a section of pantie hose, knot it and fill with flour, knotting again. They get quite messy after but we just blow them off with the blower.
Hope these ideas help. Mostly they just want to hang with their friends.