How should this sign be mapped?

Yeah, I too wasn't sure, but it's the most suitable for this occurrence.

I'm not sure if I should ask this as a new post, but would name=Colonial Knob Walkway be appropriate too?

I might add highway=trailhead to the area (including parking, etc.) See https://wiki.openstreetmap.org/wiki/Tag:highway%3Dtrailhead Then the whole area with parking and sign could be name="Colonial Knob Walkway Trailhead" or name="Colonial Knob Trailhead" (whatever the local name for that type of thing is).

And I'd put the name="Colonial Knob Walkway" on the highway=path, highway=footway, or highway=pedestrian (which ever way the walkway is tagged).

Directly using name=Colonial Knob Walkway Camp Elsdon Entrance could be fine already, as "entrance" here is shown for the trail.

@stf Since this sounds like a campsite, any pointers on where to put the highway=trailhead relative to tourism=camp_site? A trailhead can include a camp site, and a camp site can house a trailhead.