Age 6-8 covers a lot of territory... I'll make a couple of suggestions for the younger end of that range, mostly books at the normal grade 3/4/5 reading level, but which are interesting for young strong readers:

Phineas L MacGuire series, Frances O'Roark Dowell
Dragonbreath series, Ursula Vernon
Alec Flint mysteries, Jill Santopolo
The World According to Humphrey series, Betty G. Birney
The Year of the Dog (& sequels), Grace Lin
The Secret Science Alliance, Eleanor Davis
Gooney Bird Greene series, Lois Lowry
My Mom the Pirate, Jackie French
We Can't All Be Rattlesnakes, Patrick Jennings

Did Fleming Rescue Churchill?, James Cross Giblin & Erik Brooks
Tell Me a Picture, Quentin Blake

Kids Discover magazine (
The Day-Glo Brothers, Chris Barton & Tony Persiani

For readers unfazed by length:

Magyk (& the rest of the Septimus Heap series), Angie Sage -- long fantasy books that are fun and the author has said she intended them to be suitable for age 8 (so they're lower on violence, etc., than, for instance, Harry Potter)