Here is a list of all treasures in the game:
1. Dropped passport, in the entrance and exit area. You can hear this one right from where you start.
2. Golden spoon, found in the cafe on floor 1 along the south wall.
3. Clear vile, found in the world lounge in the northwest area.
4. Future key, found in the gateway lounge out in the open.
5. Baloon, found in the helium lounge towards the middle.
6. Antique watch, obtained by searching the cabins. One of the NPCs gets enraged and battles you, defeating him gets you the watch. I think you have to do this one before the ship is under attack, but it's easy enough to do with Ross at level 5 or so.
7. Silver dollar, found in the casino near the teleporter to the secret place.
8. Useless litter, found in the falcon dining area (the one the group has breakfast in). This one is time sensitive, get it before the first dream sequences or it's gone forever.
9. Ear ring, found in the dinner time restaurant on the north side.
10. Legendary hockey puck, won from the director if you defeat him when he challenges you to an air hockey match. If you lose that match, this one becomes unavailable as well.
Then there are several in the drug dealer's hideout:
11. Bronze Vile, obtained by going straight right when you enter the hideout instead of north towards the door to the second area.
12. Unknown substance, you'll notice this as you return from where the bronze vile was and head north.
13. Used needle, it's the item you can hear in the location where you found the substance but it's behind a wall so you'll have to circle around a bit to reach it.
14. Jason's watch, near the door leading to the drug factory portion of the dungeon.
Then that's it until you return from Heaven and the second dream sequences:
15. Coffee mug, found in the ship's control room but only after the attack. The control floor is the next floor after recreation floor 4. Once there head through the door on the righthand side. This becomes unavailable after the events in the employee's lounge.
16. Candle stick, found in the employee's lounge.
Then the last few are in Lower Chamber 4 (after unlocking the second key fragment door.
Gruff's T-Shirt, Jadon's wedding ring and hockey metal: explore the maze (but don't take the teleporter) and you should come across these three in short order.
Then take the teleporter and instead of going through the door to fight Jadon, follow the path until it leads to an opened room. In the corners of this room you'll find the unknown pendant and cell phone.
The other two are the black metal and the paladin employee's card, not listed here because acquired during required scenes making them impossible to miss.
If you have a total of 23 before the boss battle in Lower Chamber, then you're covered.