I LOVE these:
I seriously, seriously love them. I could probably do them all day long. I have a little book that I take to my kids' gym class (more on that later).
Here's another type. Very much the same, but it works with the picture at the bottom of the page rather than the grid:
online site that has these, but they get predictable. They're also a tad bit easier than the book I have.
Another, very closely related nerdy thing that I love is called logic art. It's kind of like the logic puzzles, but when you're done, you end up with a picture, again using process of elimination. I first was introduced to logic art when I took a nerdy extra math class in high school called discreet math. (And, I actually really enjoyed that class.) Anyway, I couldn't figure out a great way to explain it, so I scanned in the directions:
Griddlers. I could spend all day doing these, as well.
And now you know! (Um, like you didn't know before that I was a nerd. Come on.)
P.S.-There's a nerd site I've recently found, called GraphJam. Embrace the nerd within!