A good book for those of us with preschoolers is Mommy Teach Me by Barbara Curtis that talks about bringing teaching opportunities into every day situations. I'm not personally a huge fan of structured learning, especially at preschool age, but I'm a huge fan of hands on learning and reading.
Today I found a simple lesson in units of measurement using Leo Lionni's book Inch by Inch and a measuring tape. A ruler would also work fine. In Inch by Inch a resourceful inchworm uses his unique talent (measuring things) to talk and trick his way out of being bird food.
I picked out the story just to be a story, but right away I realized that Coryn might not even know what an inch was, so I read the first few pages then stopped and took out my measuring tape. First I showed her an inch, then I measured my hand, then hers. We read the rest of the book . . . and she wanted to start again, and measure our hands again, and then point to the numbers on the tape and name them. . . almost all the way up to 50.
The naming numbers impressed me because counting is not Coryn's strong suit, but apparently pattern recognition is. She would pause and wait for me to name the multiples of ten (20, 30, 40) and then say the names (41, 42, 43) of he numbers following it.
There are plenty of ways to expand on this "lesson light." You could cut out your own little inchworm and use it to measure things. You could follow up by cutting out a "footworm" and a "yardworm." I suppose you could find a way to work in metric, but I think a good deal of the reason metric never caught on was because "centimeterworm" will never be easy for young children to pronounce.