Wonder Trophy in Journey
Meet 10 or more unique travelers.
How to unlock Wonder
There are a maximum of two travelers (you and one other) at any given moment. If a traveler exits the online game, a new traveler might connect. Over enough time, you will eventually have met enough travelers to unlock this. It doesn't have to be on the same play-through. During the credits at the end, it will list the companions you met during that game. I had met eight travelers during my first online game, and early on my collectible run, this unlocked.
If you want to do this while idle, you can always just put down the controller at the start of a chapter and walk away. A great chapter to "camp" is Sunken City, where travelers will replay several times to get those symbols and the Adventure trophy.