An online friend just asked how to make her nature hikes better, so I gave her my quick list (she lives near a river):
Try a scavenger hunt. Let the big kids help the younger kids. Make some clues hard. Maybe the big kids could write the clues for the younger ones.
Similarly, you can play bingo with a set of cards keyed to the park. The kids can see who finds what to win "bingo" first. You can use words or photos.
If you have a digital camera, have the kids do a "close up" safari, with each taking a turn catching a picture of something very close. When you get home, see if the kids can tell what each item is.
Do a blindfold walk, with one of the kids leading another, taking care to keep the blindfolded one on the path, safe from rocks and roots, dry 
, etc.
Do a listening activity. Set up a blanket (very CM!!) and make that your headquarters. Have the kids listen quietly for a sound and then describe it to you.
Send the kids out from the blanket for three minutes. Each will tell you everything he saw during that time.
Make a collection (check the park rules). Categorize it.
Take water samples from the river (always have zip-lock bags with you). See if anything can be seen swimming in the water (especially in the spring).
Make mini-parks: Use a wire hanger stretched to make a square. Have the kids describe all the "wildlife" in the park. This works well from a blanket "headquarters" if you toss the hanger randomly a short distance from the blanket. Once you are surrounded by mini parks, set your rangers out to explore.
Prepare an unusual treat to be enjoyed at the destination. Watch the wildlife as you eat
