This book is so much fun. It offers 365 nature activities or ideas to explore throughout the year - one for every day. It is geared toward the very young to the very young at heart, so you can adjust it to any age.
The Kids' Nature Book by Susan Milord
Right now we are going through winter, so we are learning about temperature, snow, and everything snow related. We recently did a study on snowshoes, and my husband threw together a pair of makeshift snowshoes using twigs, wood, and duct tape.