K.N.
My daughter loved tye-dyeing at her 16th bday party. Another big favorite was a party with karaoke and wii. Depending on the weather, scavenger hunts in teams can be a big hit, too. For 16 year olds, we've found some music and food works. Pizza's fine, but my daughter tells me her favorite parties are when there's "real food", which can be as simple as pasta & sauce, salad and some bread.
It seems a general plan with room for some variation and then time to just hang together works well.
Have fun!