For a while our DS really enjoyed playing with liquid measuring cups of various sizes and colored water. (You can also get needle-free syringes from the pharmacy for smaller units.) That got the liquid units down pat for us. Summer is a good time for that project.
He also always had free access to measuring tape and things to measure.
There is a nice series of books about a dog named Penny-- Measuring Penny is the title of the one I'm thinking of for you. It's a math concept book about a variety of kinds of measurement, which my DS found engaging.
One of the few things I like about the Everyday Math curriculum our schools use is that it goes for the big picture concepts about what units are for as well as the details of how to measure: if you wanted to weigh an adult whale, would you do it in ounces, pounds, or tons? How about a baby mouse? Is it better to measure your foot in inches, feet, or yards? And so on.
We extended this to the idea of rounding error: if I round my age to the nearest 10 years (or the nearest decade) you do get a fairly good idea of how old I am; but if you round it to the nearest hundred years (century) it's meaningless. DS finds this hilarious.
DeeDee
