Hiking is free if you have access to it. As well as park. But i do acknowledge not every city/rural places have such luxury, with the overdevelopment and car infestation, greenspace is hard to come by.

I do get your point though, here in tropical country, i always have intense urge to just hike a random mountain but if i wanna climb that mountain i have to have permit and a guide, which makes sense because if not they might have to retrieve my corpse afterward. The world are actually very open, but man we human has lost that skill to unlock those place.