What I plan on doing for my possible future course (I'm gonna be really bummed now if it doesn't pan out) is hanging makeshift wind chimes from tree branches. Either made out of metal piping or bamboo. It would be enough to let you know if you hit it. If I figure something better out I can always take them down easily enough.
That actually sounds like a really good idea! I think you could hang pretty much anything and be able to see it get hit but I really like the idea of the bamboo. Let me know how it works out when you get a chance to try it.