I like this series by Candace Ransom. It's similar to the Magic Tree House in that they travel to different time lines on a mission. I like these better though. There are 3 kids and it has a fun Thomas Jefferson, Virginia connection:
http://www.amazon.com/Secret-Tower-T...rds=Time+Spies There are several books in the series.
My son likes Geronimo Stilton which is fun, but not particularly deep. I much preferred the Thea Sisters (A Geronimo Stilton spin off) because they travel to different cities and countries around the world, but they are written for girls so they have girls as the main characters. My 7 year old likes these too.
We've also read the moon gobble series which is more of a dip into the fantasy/magic type genera. They have some good boy hooks (like the magician that turns everything to cheese because he can't really do a spell). And a lot of the obstacles or monster type things they run into actually end up being their friends and helpful in the end.
http://www.amazon.com/The-Weeping-We...rds=Moongobble
My absolute favorite though is Betty Birney and the Wonderful World of Humphrey. I think this though might be more of a 5-6 grade reading level, but I'll put it out there anyway for your 'future' list:
http://www.amazon.com/World-Accordin...words=humphrey There are some 10 or 11 books in the series about a classroom pet that is a hamster and all the things that go on in the classroom and the kids lives he's with.