Some things I do that work well not only during the summer but all year long are library visits.  It's the one place I really give my daughter lots of freedom.  You want to stay an hour at the library?  No problem!  You want to check out 15 books?  No problem.  We've been going since she was crawling and I can tell it's had an impact on her.  She's very comfortable in a library and knows her way around. 

Besides visiting the library, we read fantasy chapter books together each night.  For example, we read Harry Potter's first book, each night, a chapter or two.  On an early start night, we'd read 3 or 4.  Then we read Dragon Rider.  The latest is the Chronicles of Narnia.  That's been the best!  We just finished the Lion, the Witch and the Wardrobe.  We both were almost in tears. 

Lastly, each day, I send her outside for at least an hour and she needs to figure out what to do with herself, whether it's jump rope, sit and look at bugs, or stare at the sky.  I gave her a notebook so she can write down all she finds.   At some point,  I join her, usually towards the end, and ask her about what she's found, and also either play frisbee with her, jump rope too or go for a nature walk at local trail.  I realized recently why I have so much difficulty being active.  I was never outdoors as a kid due to an overprotective father who never excercised.  I struggled my whole life with self-image and motivating myself enough to get moving.  It's been a long road but I'm in a good place now.  I want to offer my daughter that opportunity to be active as much as possible, and enjoy the outdoors, regardless.  I know it will instill in her that opportunity to rely less on "fake" entertainment and more on her creativity and motion.   I admit, I've let her watch t.v. and go on favorite websites more than when school is in, but it's all within reason. 

The best device I ever purchased?  A kitchen timer.  No arguing.  I say you get 1/2 hr or 1 hr doing such and such and when it rings, it's the cue she needs.  She just stops.