This guide is not quite complete yet. Spawn limits for some POI are missing. Anything that is not verified is marked with (?). Let me know if you have more information on those or have anything else that you'd like to add.

Please point out any errors you find.

POI's come in 3 rarities: Green < Yellow < Red. POIs of higher rarity offer better rewards but are guarded by stronger enemies.
Higher rarity POIs spawn further away from the shop.

XP values are for Nightmare difficulty, where all Xp gains are reduced by 30%. If you play on a lower difficulty you will get more xp (Adventure Xp = Nightmare Xp / 0,7).

5 per floor

Fully heals you and grants +10 Vitality.

2 per floor

Take damage equals to half you max hp to gain +5 Dmg. Cannot kill you.

1 per floor

If a player is fully dead you can revive them by spending 50 gold per player.

If all players are alive you can buy a feather and some xp. You can only buy one feather per floor.

If you have already bought a feather, you can instead buy a greater amount of xp. You can buy xp repeatedly and the cost does not increase.

0-1 per floor

0-1 per floor

Clear 2 waves of enemies to gain 2 keys. The enemies depend on the current floor. If anyone dies during the ritual all enemies despawn and the ritual is cancelled. You can retry an unlimited amount of times.

Choose 1 item. Higher chance for high rarity items with increasing chest rarity.

1-2 per floor

Use a key to challenge an optional boss for an Legendary or Cursed item + XP.
For every floor there is 1 boss that resides in a cave and 1 that is fought on a timer. The cave bosses drop a mix of Legendary and Cursed items. The timer bosses always drop exclusively cursed items.

0-1 per floor

Use a key to obtain 2 copies of a new item. Rerolling costs 2 Stars of Fate instead of 1.
The POI symbol on the map can be a bit misleading, pay attention to the color of the 'X', the chest symbol itself will always be displayed as yellow.

1 per floor

Defeat 3 waves of enemies to earn XP + reduce Master Nightmare Hp by 30%.

1 per floor

Complete the quest to get a Legendary item + Xp.

0-1 per floor

Refugee camps always contain 3 different refugees, all of which have unique trades. If no refugee camp spawn, corpses can be found lying around instead.

Refugees are a lot more complex than other POIs and covering everything about them would take an entire extra guide. Heredos did just that, so check it out:
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3457697715

1-2 per floor

Reveals POIs on the map and grants XP. There is no red rarity of this POI.

0-1 per floor

Thanks to Norman for providing a XP spreadsheet that not only covers XP of all enemies but also XP rewards of POIs! All xp values are taken from that spreadsheet and reflect Nightmare difficulty.

Thanks to Andrew, Pam, Tosheros for providing info on some POIs!