Stand in front of a barrel, then press X or Circle to turn it into items, such as string, red legion, rock sugar, fest rock and more.
Bonus Dungeon and Arlin
After beating Amalgam, watch the ending then save the bonus data. Load game, go to your base to trigger a scene about a new dungeon in Ka Luda's Playground. Now go to Ka Luda's Playground, use float ability to find a new portal on a boulder near southwest entrance to reach Mull's Dungeon. You'll also get Arlin to rejoin your party in Mull's Dungeon.
Bonus items
To unlock a bonus at the main menu, go to the Collectors Caravan and talk to the collector named Lector. She will talk about items that you have acquired. Then, Popo will lecutre you about the item list. When he is done, talk to Lector once again. You will now see the item mission that you have completed, or the right items you have collected. Once you have finished one of the missions, talk to Lector to unlock an introduction or picture in the bonus menu.
Delsus Spirit Attack
Before heading to Arcose when they open the mines. Talk to the old man in Lake Forwell. Have Gogo Suit and Snickerdoodle ready to give to him.
Klein's Living Item Attack
Once you've spoken to Zeldalia, speak to her again. She will give you the choice of Living Item or a Mana Item. Choose. You'll get the other later on.
Klien's Background/Picture
Collect "Klien's Cane" and "Medicine" and show them to Lector, this completes the "Travel Kit" series of items.
Norn's Illusion Attack
In the playground, during or after you do the test in the Playground, arrange all the purple pieces on all the squares (all 4 corners and the center)
Unlocking Bonus material
To unlock Bonus Material that can be viewed at the Title screen you need to show Items to Lector who resides in the Collector's Caravan. The more items you show her, the more chances you have onm unlocking Art Galleries, Movies, Character Backgrounds and more.
Barrels...