We have done the Family Math program (sponsored by an oil company I won't mention here - but it's in the link below!) through Ontario Early Years Centres at our local library a couple of times, and I would definitely do it again.
If you don't have this program available locally, or want to try a few easy, informal math activities on your own with very young children, there are a lot of very good resources to download, free, at the Family Math site:
The Early Years Homebook is a great starting point, but it's meant as a complement to the classes. If you haven't done the classes, maybe take an evening to read through the Early Years Resource Book to get some ideas for beginning math with young kids.
The first time we did the program, it was presented the way it's meant to be done, as a series of six weekly sessions. The second time, in August, the six sessions were condensed into five consecutive afternoons. That format didn't work nearly as well; the six-week program is very well thought-out, with a different theme each week (bugs, buttons, rocks...); as a five-day program it mostly just felt "jumbled". I also didn't like having to do homework every single night.
Anyway, I thought I'd post this here because these are terrific, professionally-developed resources (they even have a bunch of "math songs" for free download on the site!) that I for one definitely plan to use over the course of this JK year.