>>11
Please don't tag any sites. One of the main generally accepted rules of UE is to "leave only footprints" - i.e. don't disturb the sites you visit in any way, shape, or form. Trespassing is already a crime, but one should respect the property so that the owners won't have any reason to call the police or install heightened security. Even leaving something behind can be considered littering, so it's not a very good idea.
You may want to look into Geocaching instead. It pretty much goes along your idea; hunt for something somebody left behind, leave your "mark" (usually a small trinket) for others to find. Also probably not as dangerous as UE, and can potentially be done solo, depending on the site. I'd do this but I haven't the money for a GPS unit right now, sadly.