You can use heavy duty trash bags-the kind used in trash compactors, or if in a park, they may hand out "blue bags" which are similar construction, also called wag bags. It is a bit tricky especially if you are fully dressed, but it can be done. Another reason to carry a shovel-to make yourself a latrine which you can line with the bag. If you want to get fancy, I suppose you could carry in one of those little camp toilets that like a folding chair or stool. Same principle.