I went nearly every summer as a teen and I was never bored, but I have always loved the magic of Disney. My favorite parks as a teen were MK and EPCOT. I couldn't really spend more than a half day at MGM. I didn't go to AK until I was an adult, so I don't know if that would be boring to a teen or not. We absolutely loved the waterparks, too!
My "supervisors" (never went with my parents) usually let me wander around either with a buddy, with my brother, or in a small group. When I was older (17-18) I was allowed off on my own, but with specific meeting times and places. If I failed to check in within 10 minutes of the set time, they were going to assume that something was wrong. (Needless to say, I was usually early to avoid getting in BIG trouble!) Of course, it really depends person to person... they wouldn't let my brother go off alone because he can be irresponsible.
Good luck!