When working outside, I end up using a lot of different surfaces. A stock item in my bag is an old white sheet. I have various sorts and sizes from king-size (ideal for whole class work) down to cot-size (for pairs or individual work). I use them deliberately to highlight natural materials.
Without a plain backdrop, often the beauty and detail of a design can be lost. Or children simply cannot see what you are trying to show them.
The sheets are very prone to getting wet and muddy. I try not to wash them unless absolutely necessary, otherwise I think my washing machine would be constantly on.
For really foul weather, I flip to plastic sheets. In the photo below, I have used magic whiteboard sheets. When I purchased these almost four years ago, I thought they wouldn’t last long outside. They seemed really flimsy. Over the years, I’ve found they lose their static and are no longer ‘magic’ in terms of sticking to walls. The sheets have grown thicker and tougher and have become more robust with age.
I find on windy days, either the plastic or the cotton sheets do need weighted down at the corners. In fact, sometimes, this adds an additional problem solving element to any activity!
The only other thing worth remembering is that on snowy days, a white sheet is simply not needed! Bring on the winter weather!