Once I have one more officer I will start details. Here in Texas we have a number of Historical Sites that are overseen by the State park system but are not state parks.
One of the more famous is the http://www.thc.state.tx.us/museums/musnimitz.shtml National Museum of the Pacific War which is managed by the Texas Historical Society. This surely does not fit under signs of history or any other I can think of.
These would require more than a sign or marker. There must be something there that is open to the public.
Another is the Texas State Railway which was once a State Park but is now operated by a private company.
If I can identify 20 or better sites here in Texas that are listed under the state park system I would imagine that most all other states and countries would have similar types of historical sites.
I think the uncategorized waymark thing is just a reason to start a new category.