Hi Susan,
My girls are 8 and 11 also. I am well acquainted with all things tween girl at Disney. If you're specifically looking for things to do with your 11-year-old, you might want to check out
this post where I interviewed my oldest daughter about the things she likes best at WDW.
In addition to the items mentioned there, I'd certainly add the High School Musical Pep Rally, which plays several times a day at Disney's Hollywood Studios. There's also a Hannah Montana makeover at the Downtown Disney location of the Bibbidi Bobbidi Boutique that's getting rave reviews.
As for attractions, your girls are likely tall enough to do almost every ride at the parks. My girls have become quiet the roller coaster addicts. I think that it makes them feel cool that they're much braver on Expedition Everest than I am. (
Why don't they scream? I can't ride it without screaming. It's like a reflex or something.)
On my most recent family trip to Disney World, I felt like my girls started taking a lot of pride in acting "grown up." The more advanced rides and the fancier restaurants really got them excited. So beyond the tween things (like the HSM Rally), try a night out at a signature restaurant like the California Grill. I bet that would be a big hit with the mature young women in your house ;-)