These are all the treasures not currently listed on the front page
... Sorry if they are mentioned elsewhere in this thread.
Mithalan Banner - There are eight rooms by the entrance of the Mithalan palace - four above and four below. In the top-left one, there is a blue banner - this is it.
Krotite Armor - In Home Waters, go towards the song cave but take a left and you'll end up at a pearl-locked gate. Unlock it and enter the shaft in energy temple- atop this shaft is the Krotite Armor
Tooth - You get it from killing the Energy god - that is, the huge fellow in the basement of Energy Temple. He drops the tooth on the ground when he dies, though it can be hard to see if you're not looking for it.
Sun Key - In the sun temple, along the eastmost wall of the first place where you'll have to manipulate the water level, there is a hidden passage (look on the minimap). The passage is in the first shaft in which you lower the blue crystal.
... And finally...
The "Ceremonial Costume" is in fact called Mithalian Dress
The prince's weapon is called Drask's Trident