I am currently on my H1B status and my visa got expired already. In this scenario, can I visit the park (not crossing the border) and then go back to Seattle? What documents do I need to take to visit the park?

If your Canadian / American bring your expired passport, citizenship, driver's license etc. The agents just want proof your living in Canada / United States. You can meet with family / friends / partners and will be required to show your documents when returning.

I visited the park, didn't attempt to cross to Canada and wasn't asked for documentation when I left. There were lots of boarder agents around so not sure if they sometimes ask for id

Yes you can,And meet your favorite Canadan