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:

Fiction:
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

Non-fiction(ish):
Did Fleming Rescue Churchill?, James Cross Giblin & Erik Brooks
Tell Me a Picture, Quentin Blake

Non-fiction:
Kids Discover magazine (www.kidsdiscover.com)
The Day-Glo Brothers, Chris Barton & Tony Persiani

For readers unfazed by length:

Fiction:
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)