My daughter is almost four and is just now getting interested in workbooks or formal learning. We are using the Rod & Staff's Preschool series($9.00 for set). My son used this series as well, so it's been fun to revisit it. Great memories! These are wholesome "old fashioned" black and white workbooks published by conservative Mennonites. Simple rural living features prominently in these workbooks. When you click through to the link, each individual book can be viewed for content.
For older preschool children and kindergarten:
More preschool posts here.