Summer Sketch #1 - catalog characters
Look through some old housewares catalogs and add your own cartoon characters in the space using a sharpie.
Summer Sketch #2 - shadow tracing
This works best either early or late in the day when shadows are longer. Find an interesting shadow and trace it directly on the concrete with chalk, or place large paper on the ground and trace with pen or marker for a longer lasting work of art that can later be painted. If no paper and paints, snap a photo of your chalk drawing for your sketchbook.