Reference

Some treasures are cameos to other Nintendo series. These include:

The Flame of Tomorrow exhibits the text 'Bowser Breath Brand' with a picture of Bowser.
The Master's Instrument says 'Mario paint', a possible reference to the game Mario Paint.
The Decorative Goo, bears the label 'Mario Paint--Blue'.
The Unspeakable Wonder resembles Princess Peach's crown.
The Cosmic Archive is a Japanese Famicom disk with the game Nazo no Murasame Jō.
The Dream Architect is a Game & Watch.
The Implement of Toil says 'Nintendo 64' on it.
The Remembered Old Buddy is the head of a NES R.O.B., the gyro blocks being R.O.B. blocks.
The Gyroid Bust is, although fairly altered, featured in the Animal Crossing series.
The Key resembles the secret exit key found in Super Mario World.
Pokos resemble coins from the Mario series.

Many Pikmin 2 treasures are real-world items, with real-world brands. For example:

Dr Pepper is the drink of the bottle cap known as the Drought Ender in-game.
The Abstract Masterpiece is a Snapple lid.
The Gherkin Gate is a Vlasic brand lid.
The Salivatrix is a Dannon yogurt lid.
The Quenching Emblem displays the 7 Up logo.
The Pondering Emblem (NTSC) and Massive Lid have the brand 'Yoo-hoo'.
The Alien Billboard is a Kiwi brand shoe polish lid.
The Boss Stone is a gyokushō piece from the Japanese board game Shogi, also called 'Japanese Chess'.
The Yellow Taste Tyrant is a French's mustard lid.

Other such cases are the items in the Mystical Energy Series, which are all Duracell batteries.

See also