Definitely think the first option would be really cool. It would have that "see that mountain, you can climb it" it feel but with good climbing, like Shadow of the Colosses where the large structures (much like they are in the place of the colossi) are where there is chanlange, but there can also be fun in climbing arould the world between them.
Oh and what could even lean more into that interesting world is events that happen only at certain times, ones that you might stuble into if you are really lucky or more likely you find hints to when and where these things happen, such as a door that only opens are just the right time of day or moon light at the right phase of the moon. A more amitious kind of event could be some floating structure that only gets close to some towers at a certain time.
I'd say the third option could also be cool, though it gives less freedom for exploration, not just the freedom of choosing which montain to climb, but also exploring things that isn't marked and could be nothing, but could also be some cool place to climb up.