DS likes rocks and Greek mythology - I guess those are the only "out there" hobbies. Well, he is just now developing an interest in intricate paper crafts - he made an incredibly detailed model of a charcoal grill for his dad's birthday a couple weeks ago (complete with hinged lid and tiny utensils hanging off the side!), and before that he made an elaborate house/yard scene entirely out of paper. I hope this interest continues for awhile - it seems to be a good way to exercise his creativity, problem-solving skills and patience but it isn't overtly "smart" so he'd be less inclined to hide it like he has done with more academic interests.
He liked sudoku around age 5 too, but I don't think he ever did the full-size 9x9 puzzles.
