I don't know if anyone has seen the "chocolate math" thing that's been circulating via emails and other places (google it, it's the first thing that comes up), but it might be fun for your kids to try to figure out how it works.
I didn't really know where to post this, but since I chose here, I should probably add a brag to this...
DD14 enjoyed that, although it didn't take long at all, I'm sure it would be more of a challenge for younger kids from about 8-10 or so (having said that, *I* probably wouldn't have been able to figure it out very quickly or at all, and I'm in my 40's.... but I'm our family's non-gifted person)... Anyway, after she figured out how that one worked (which was at most 5 minutes), she went to work to create her own trick of the same sort. Not too big of a brag, but still kind of a fun story, I guess.