Feb. 25, 2006 - Magic Treehouse Books... What next?

So, for those of you whose children love(d) the Magic Treehouse Books... what would you recommend for the next reading level (or two, actually) that might interest the MTH fan? 

My son just flies through those books now.  He picks one up when we stop by the library for a short visit and immediately is immersed.  He often has his finished by the time we get home from the library!  And we only live a few minutes away.

He is very interested in all things science, animal... and time travel, ancient history stuff and fantasy.  His reading level is somewhere between 4th and 6th grade, depending on what test we use:)

Ideas?