Most people I've talked to love the program our district has for HAG grades 3-5.
I think this site put together by one of 3rd grade teachers gives a pretty good summation of what they strive for:

http://wsfcs.k12.nc.us/Page/21041

The classes are self contained and very project based with a lot of student interaction. They also include enhanced History/Geography studies and Spanish. The basic curriculum is also enhanced and gradually accelerated to around 2 years ahead of grade level. After school clubs include Lego Robotics and Chess.

Hope it helps with your research.