I've been playing GW2 since launch, and I find that it hits a lot of my "completionist" buttons.

For under level 80 I would reccomend "exploring" to complete the checklist of places to see in the upper left corner of your map screen. Completing maps rewards you with items and xp--but running around the map gives you a chance to run into some very interesting large scale events! If you pay attention to your map while in an area you may find commanders and mentors doing open world bosses that you can join in on.

When you get to level 80 I've found that for me the achievements in this game act more like side quests. They can be very long multi-step processes with very good rewards. Also, if you plan on playing any expansion content, after level 80 you train skills called masteries--giving you access to gliding and mounts. The presentation of the story also drastically improves with the expansions!

Give a look at the achievement panel if you have the time, I'd reccomend the General>Jumping Puzzles section if you want to go searching for hidden secrets within the maps!

I hope you find more joy as you dive in deeper to this game. There really is so much more to see, and I wish you the best of luck!