At 2, DS enjoyed the Kumon "Lets Color" and "Lets Sticker and Paste.". Now, at 3, he likes the Kumon mazes book. We also sometimes work out of the Critical Thinking Company's "Mathematical Reasoning" book. I like that it is more of a guided discussion versus a workbook.

http://www.criticalthinking.com/getProductDetails.do?code=c&id=06913