All good points, I can admit Firewatch isn't for everyone even though it's one of my favourite games of all time =)
The only other game I've played where the gameplay feels almost as 'free' and open world as in A Short Hike is Sly Cooper (any of the games from the series). You can climb around pretty much anywhere, there are ways to glide around (if you get a 'paraglider') and an open world full of opportunities to jump around, explore and climb. There are hidden treasures to find, too. The game is very based on missions and doing quests, though, and the environment is definitely darker and more action-packed, but the exploration feels very nice in this game.