I would have outdoor things available but not structure any specific activity.
Like several others mentioned, have a stash of sidewalk chalk, bubbles, a ball or two of some sort to kick around or toss around, and then don't worry.
Kids are amazingly creative--when we let them be.
I would not plan on anyone having to march them to and from the park, either... And I love the idea of "playing outside today" notes on the doors. I, too, have hosted get-togethers where stuff magically was broken or ransacked. Then it is a lot of work for the kids to try to put their rooms back together again. Really not worth it. If it was one or two kids, then fine. but 15?? No way. That's inviting a ransacking, in my opinion.