I used to sit for a couple with a 4 year old and 6 year old. One of their favorite things to do was to go outside and find bugs, then look for those bugs in their books and learn what they were called. They had one of those little bug catcher things so they could sit and watch them easily. While a few critters did unintentionally get crushed, the girls learned a ton and loved playing outside. We did the same activity vice versa as well.. looked at bugs, birds, etc in their books and went in their backyard and looked for them there. The girls learned about flowers and plants the same way. It was fun. They had a huge backyard too, so it was pretty easy for them.
//long winded