Since I typically do programs at a place where other activities are happening, I schlep a lot of stuff around. This helps me remember everything I need. See some examples of the results of this under "Nature Preschool" on the blog.
I use a few resources to help in planning:
Growing Up Wild
Environmental Experiences for Early Childhood
Here are a few examples of how our activities turned out. Honestly, there is usually enough that could work for a week at a time; however, we just had one morning a week for a couple of hours.
A Natural Holiday
What's Bugging You?
Here are some events planned using a similar sheet:
Fairy Wings and Wild Things
International Mud Day
Find the planning sheet here. Let me know how you use or modify it--I love seeing how others use similar stuff!