We've read a lot of different chapter books to the kids at bedtime but one of our favorites are The Magic Treehouse series by Mary Pope Osborne.
The boys love them, they have adventure and mystery but aren't too scary either. I like them for numerous reasons. We can finish one in about 3 nights, it helps the boys work on their listening comprehension skills and they learn about history as well. I have a few of these books on CD that I play in the car, which is also a bonus!
Geronimo Stilton and Junie B. Jones rank pretty high up there around our house too, although I know as soon as they find out about Captain Underpants he will rule!