It's tough when you have an advanced reader--they are ready to read at a higher level, but the content of some of the higher level books may not be appropriate. My 8 year-old is reading at a 9th grade level and sometimes that can be a nightmare, lol. He isn't interested in any of the Magic Treehouse, Cam Jansen, Geronimo Stilton stuff. He's way past Captain Underpants, although he does (like all kids) love the Wimpy Kids books.
My son has read at least 60 of the Goosebumps books and loved them. He also likes a lot of non-fiction stuff, like books on sports, weather, natural disasters, etc. Like someone else said, my son loved the Stink books. He also liked the Roscoe Riley books. Oh, and don't forget to try Fudge/Superfudge by Judy Blume!