This one can't be missed. As you approach the Grumblebum Thrasher, you'll see it on a hill to your left.
This one's hard to miss too. Continue forward to spot it down the path.
Before leaving Snagglescale Swamp, you'll pass this Elemental Gate. Use a Life Skylander to open the way, then complete the fishing minigame inside to win a great prize.
After riding the tube the first time, make a right just past the missing gear. Follow this narrow path to find another preserver.
Hang right upon entering Broken Bog Bay to approach an Elemental Gate. Use a Magic Skylander to open the way, then complete the platforming segment within. The treasure awaits you at the end of the course.
Once you grab the gear inside Big Gill Water Mill, don't exit. Instead, hop across the nearby gear to reach this little alcove. You'll find another preserver inside.
When you reach Muddy Marsh Village, move right and hop along the boards behind the houses to reach this.
In the north end of the village, a Boghog (one of those green boar things) is asleep. Bounce on it a few times, and it will burrow away, digging up a collectible in the process.
In the same section of the village is a dual Water/Undead Elemental Gate. Use a Swap Force character with these elements (or two appropriate Skylanders, if you're playing co-op) to open the way forward. Complete the battles inside to earn this.
While destroying the crystals that have imprisoned the Flashfin, look to your right after using the first launch pad.