My son is like the president of the Magic Tree House fan club or something. He's obsessed. I've have worked with him on taking an interest in other books, since he's read every MTH book 5+ times, and here are a few series he's really liked:
Time Warp Trio (a LOT like MTH)
Oliver Moon
Franny K. Stein
Flat Stanley (might be perfect if you're looking for something a little easier than MTH)
Secrets of Droon (I really liked this for my own son because the books jump a reading level -- they get harder as kids go through the series, so it sort of gently challenges them)
Captain Underpants (I don't know if you want to go there; I had/have misgivings myself, but my son loves these)
For nonfiction series, I also recommend trying the MTH Fact Trackers and the "Who Was" biography series.
Finally, one "big chapter book" my son loved was Three Tales of My Father's Dragon. The reading level is very similar to MTH, and it's sweet, beautifully told, and fast-paced -- it's a great intro to reading a longer book in multiple sittings.
I hope this helps. Let me know if you want authors' names or more specific info/details on any of them.