Another simple activity that uses fractions and conversion ratios (ultimately the basis of dimensional analysis) is cooking. Scale a recipe with a kid, and they learn an awful lot about working a word problem, and with a lot less angst than usual, particularly if the end products is... er... cookies.
"Uh oh. I don't think we HAVE that much oatmeal. That's probably a good thing, though, since I think we'd be pretty sick if we eat ate two dozen cookies. Do you think we could figure out a way to only make half that many?"
