I remember so many of these books fondly from when I was a child (we homeschooled). My Dad read The Lion, the Witch and the Wardrobe to me when I was 3, and after that we would go on hikes and pretend we were "looking for Narnia."
Some others that we really enjoyed were:
Treasures of the Snow, The Tanglewoods Secret and everything by Patricia St. John
Little Britches, The Home Ranch and others by Ralph Moody
Brighty of the Grand Canyon, by Marguerite Henry
Gentle Ben & Kavik the Wolf Dog, by Walt Morey
Snow Dog, Wild Trek and all of the wilderness adventures by Jim Klielgaard