In this page, I will post everything related to background information in the Pikmin series (rests from the human civilization, for example). I will try to classify them (decorations, machines, and so...)

Made by humans

Christmas decorations

These appear in the Fortress of Festivity, in Pikmin 3.

A festive cone. It resembles the Boom Cone from Pikmin 2.
A bell. It is similar to the Danger Chime in Pikmin 2.
The different baubles which appear in Pikmin 3. A bauble called Essential Furnishing also appears in Pikmin 2.
A tinsel.
The different balloons appearing in the Fortress of Festivity.
A red candle. A candle also appears as a treasure in Pikmin 2 : it is called the Fleeting Art Form.
A snowflake

Gifts

A gift appears as a collectible treasure in Pikmin 2 : the Joy Receptacle. In Pikmin 3, there are a few gifts that appear in the Fortress of Festivity : they aren't collectible. Here are a few montages showing these gifts :

Tools

A fork and a knife.
A teaspoon.
A spoon in a pot.
A montage showing the different screwdrivers found in Pikmin 3. The green and red screwdrivers can be found in Clockwork Chasm, while the blue and yellow screwdrivers can be found in the Corroded Maze.
A pair of diving goggles.
A snorkel.
The different buckets appearing in Pikmin 3.
An orange sieve in the Forgotten Cove.
A blue sieve in the Forgotten Cove.
A shovel in the Forgotten Cove.
A rake in the Forgotten Cove.

Unclassified

A glove which resembles the Five-man Napsack.
A tyre. Tyres appear in Pikmin 3 and are found in the following areas : Stagnant Sea, Sandpit Kingdom.
A ball.
A montage showing the control valves appearing in Pikmin 3. The white valve appears in the Corroded Maze, whereas the others appear in the Clockwork Chasm.
The two different tiles appearing in the Fortress of Festivity.
A traffic cone.
A gutter in The Rustyard. In Pikmin 2, it's possible to find gutters as well (in the Shower Room, for example)
A wall made of bricks.
A padlock.