The MO Dept of Conservation has divided the state into regions. You are in region 3. That means that you will be able to find activities and ideas from them specifically for you. Here are a few things for your region:
Call your local conservation office (816-622-0900) and ask to talk to the education person. Ours has an education agent who works with me to set up specific things like hikes to observe specific things. She also has tons of ideas and resources to help me as a teacher and group leader.
Your KC DoC office has these events already scheduled, and I have noticed that if I ask they are often happy to do these things specifically for my group separately from the open public event:
http://mdc.mo.gov/events/region/03/heading/Kansas%20City
Here is a great list of all the conservation areas in the KC region:
http://mdc.mo.gov/atlas/region/kc
Some will work better than others, of course, but if you had this list while talking to your education agent, you could probably come up with more ideas than you can use.
If you will look at the MO DoC kids magazine (free to MO residents, just subscribe), there are tons of fun nature activities and crafts to do. Many would be fun in conjunction with your field trips.
The MO Department of Natural Resources also has tons of things if you know where to look. They are doing some cool things for Earth Science Week in October 2013.
http://www.dnr.mo.gov/geology/education.htm#videos Scroll past the info on the earthquakes to see the info plus videos that they have.
Tour geological (rock) museums
Tour rock stores to explore and have some one explain what they see.
Tour a working mine
Visit and tour a cave
Hike a conservation trail
visit a river/creek/pond/lake and see the various forms of life
animal refuges and parks
zoo
bird aviarys
Volunteer to do clean up in a river section or other wildlife area
Visit a park with a scavenger hunt list and cameras (and trash bags)