I agree with Steadman here. It's important to know what kind of "outdoor" experience you already have before giving suggestions. Without much else to go on, I would say find a state park that is lakeside, hike around the lake for a half hour, hike back and let the kids play in the water while you fix lunch. Make it as enjoyable as possible for the kids and they'll remember it.

Plan on carrying any of them who can't walk yet, and spread the responsibility out among the older children. Make up some games for the hike (who can see the most birds? Name that plant (take pictures and identify them in a book at home).

When we know your hiking experience we can help more. But have fun.....!