My son is really  into mysteries right now but he doesn't like them to be scary. Ghosts are OK as long as they are good ghosts rather than bad ghosts. Treasure hunting is good. Not really into the whole detective type story like The Hardy Boys but more like the Boxcar Children books. Appropriate for a 2nd or 3rd grade reading level. Any suggestions?

Has he tried any of the A to Z mysteries....they are kind of detective like but the kids investigate things like a strange man they think might be a vampire, there is another one about Zombies,  the school skeleton is stolen in another one...my second grade son liked them a lot when we read them earlier this year....they are not scary at all. 

Thought I would add another series my son likes is Dinosaur Cove by Rex Stone which is probably more adventure but thought I would throw it out there.

Try the Cam Jansen Mysteries by Adler...very easy reading.

A to Z mysteries by Ron Roy

Jigsaw Jones mysteries by Preller

Encyclopedia Brown?  Or is that too detectivey?

The Happy Hollisters are awesome, written in the 1950s but very "safe" for little readers. They pop up on PBS every once in a while, and are still pretty affordable (except for a few rare titles) on Amazon. They are a family of 5 kids who have a mystery adventure in each book, and each one is in a different setting, like a dogsled race, a river trip, a camping trip, Florida, Mexico. Great for  reading aloud too.

The Bobbsey twins are good too, also very innocent and non-scary, yet entertaining.

I have found that looking for older kids books often helps when avoiding scariness. Hope this helps:)

arthur chapter books a lot of them are mysteries

cam jansen

jigsaw jones

clue and clue jr

scooby doo especially collect the clues you get to try and solve it

a-z

I have some on my shelf.

A to Z Mysteries

Cam Jansen

Arthur Mysteries