One of my favourite additions to Final Fantasy XIV over time has been the randomly generated dungeon Palace of the Dead.
I actually like it specifically because it’s one of the few pieces of content in the game that can legitimately be run solo while it’s still “relevant” to you. Other dungeons and Trials in the game only really become soloable once they are so far beneath your character and item level that the only reason to run them is “for fun” or for the sake of their story, but Palace of the Dead is pretty much always useful for something or other, be it levelling an alt class or simply obtaining some endgame tomestones.
The other nice thing about Palace of the Dead is that it’s been specifically designed with soloing in mind, since it even has its own leaderboard for solo adventurers.