One of my favorites (you don't even need kids for this one!) and one that works in the city, suburbs, or country. Make a list of shapes and put them on a chart: Heart, circle, square, letters, crescent, etc. and make a copy for all hunters. Then have everyone grab a camera (you don’t need Leicas for this, old digital camera, phone cameras, whatever is on hand will work) and head outside.
You need to find the shapes in surprising places and snap a pic, whether it’s a heart-shaped stone, a piece of string formed into a circle on the sidewalk, a crescent-shaped shadow created by trees. Find the shape, check off the chart. This is fun, and a great lesson in seeing the world in a different way. And the photo collection afterwards is an awesome arty summer souvenir.