Other than stronger monsters, what else can I expect during the night? Are there treasures/shops/npcs/quests only available during nighttime?
There are no special shops or treasures which only appear at night.
There are two other nighttime specific things, however.
- Certain bounty hunts only appear at night.
- Certain fish can only be caught at night.
When you accept a bounty wherein they only appear at night, you also have the option to wait until nightfall, at the Tipster, before tracking the bounty. This means you may have more of a challenge getting to the bounty.
Nighttime fishing is generally safe since Noctis is on a dock and fishing spends much time.