We are definitely a picnic family. It's so easy and the kids love it. Any meal, really - if it's a beautiful day, we spread a blanket, load up a tray with snacks/sandwiches, and sit out in the yard. The kids love it and it can lead to some impromptu lessons on nature as they sit and take in their surroundings while eating.

Load up a day pack with water and snacks and take the kids for an hour or two nature hike.  Sometimes we like to just take it easy, talk and unwind while strolling on some open space near our house and sometimes we hike with a purpose.  Give kids an age appropriate scavenger hunt or breakup into teams and see who can find the most items in a set time.  If time permits take turns at each trail intersection picking a direction, this is a good way to discover new routes.  One of my favorite things to do is hike in the trail at different times of the year or of the day.  You can see which animals are out early in the morning but asleep when the sun goes down.

Just a word of warning, you kids will adapt to hiking much faster than you.  A day hike that once left them ready for a nap will quickly grow into an adventure that leaves you exhausted. 

Give them a camera to take on a hike.  There are relatively inexpensive digital cameras made for all ages now.  My children love to take pictures when we go for a hike.  It's so much fun to see the hike or outing through their eyes.