After reviewing the options, I think guidepost is the most accurate descriptor. Other options I looked at:
- Information/information board: While this does have the purpose of marking the park for information purposes and pointing to an informational website, a) it's not in board form, and b) the information you get from the marker on its own is limited to its name. That makes it more useful for guiding than as an information source.
- trail marker: This isn't used for a trail, and it doesn't look like any trail markers I've ever seen (nor what is on the wiki)
- marker(post): This looks like the most viable alternative to me, but I feel like the rest of the marker feature types are more dedicated to infrastructure marking (like pipelines). 

I'll stick to guidepost for the rest of these park marker things.