C.M.
Chalk, we paint on the patio (on paper) and sometimes on the concrete with washable paint. Do you have a table for them? I love letting them use playdo outside since it's easy to clean up! What about a sand 'container' so it's small enough that it won't be a total disaster but not too small? You can do the water table when it's not super cold but just add warm water (and dish detergent for bubbles). We sit on our deck and play ball (catching and throwing or rolling or one of us uses a collander to catch the ball in it). Read books outside on a blanket and pillows. Do you have a tent? Pitch that up for a day.