I love the The Siblings' Busy Book, written by Lisa Hanson and Healther Kempskie. Here is the unique and great thing about this book. For each activity in this book, they share ideas for doing the activity with baby age, toddler age, preschooler age, and school-age children. Each activity has ideas for making it appropriate for these various activities.
For that reason, it makes it a great book for mothers of more than one child. But it also means it is great even for mothers of just one child because the ideas will grow with your children! This book is most defintely worth getting.
In case you don't know what a "busy book" is, it is a book that lists activities to do with your children. There are art projects, games, things to do on rainy days, pretend play, music activities, movement activities, outdoor activities, family activities, kitchen activities, and seasonal/holiday activities.