One simple activity is to have a small jar of coins on the kitchen counter. For lunch or a snack, write out the menu, restaurant-style with prices marked (under $1 to begin) and ask them to count out the correct change.
Quickly they will catch on to that, so for dessert, add a cookie, which helps them learn to add money mentally.
As they get older, you can make the amounts harder to figure by adding state sales tax, and finally a tip for the "waitress mom".
From the kitchen, your money lessons can move to the grocery store, where you allow them to purchase a small piece of candy or gum. Learning to figure the exact amount, including tax, challenges their mental processes.
Here is another fun resource utilizing the poem "Smart" by Shel Silverstein:
http://www.fi.edu/pieces/knox/smart.htm
