Whatever you do make sure its not construed as littering. That is a nice fine I wouldn't want to pay for.
Yes, this^. Don't use any artificial things like trash, tires, or pallets that could be viewed as illegal dumping. Hell, even relocating dead trees or branches might carry fire hazard issues.
If you use paint (and I'm not saying you should or should not), don't paint words, letters, or symbols - that could be mistaken for graffiti or tampering with construction paint symbols.
maybe something non permanent like those fluorescent colored flags used for the flag game could suffice?