Black bass: It's time to go fishing! Swimming about in the lake is a group of black bass, a couple of which are of the large, very valuable variety. You can either shoot them from the dock or once again hop back into your dinky little boat and use your seemingly-infinite reserve of spears to kill them. While the spear might save on bullets, though, it's wildly inaccurate and more than a little infuriating to use against such dainty, nippy adversaries. Whichever method you choose, you can use the boat to collect them once they are dead. If you have the patience, there's potentially a lot of profit to be made from gathering all of the dead fish in this area. It might take multiple trips to the nearby cave Merchant to get rid of them all, but it's well worth it.
Speaking of cave Merchants...
It's so very easy to completely miss this secretive cave-like home of the next Merchant. To reach this well-hidden cave of wonders, you'll need to once again rely on your boat to ferry you over to the opposite side of the lake. Within a part of the long-stretching hills and mountains will be an entering to a secret cave, illuminated with burning-blue torches.
Green Gem: Upon exiting your boat, walk up the steps and take a left before entering the Merchant's shop. From here, you'll need to push the large crate out of the way to reveal a once-obscured passage to the roof of the store. Continue onwards, pushing aside all further crates that stand in your way, and smash open the wooden barrels on your way. One of the wooden barrels will contain the Green Gem.
Spinel: Whereas you'd previously took the left option when standing in front of the Merchant's shop entrance, this time take the right. Again, you'll need to push a huge crate out of the way. Once you've done that, head around the corner and shoot the hanging torch. The Spinel will drop to the floor and will be all yours.
5,000 PTAS: Murderize the Ganado with the parasite for a head.
Spinel: Follow the waterfall stream along for a bit and climb down the rope to reach the lower level, as you would normally do, but pull a 180 when you reach the bottom and look upwards just a wee bit to discover your next Spinel treasure sticking to the mountain.
Amber Ring: After shooting down the suspended crate and using it to cross over to the second walkway, head nearer towards the center of the waterfall; as you near it, you might notice a sparkle come into view as you approach the overhead wooden beam. Shoot down that magical, shimmering thing to earn yourself a fancy Amber Ring.
Spinel: Once you have picked up the Round Insignia and exited through the door that leads to dock with the motorized boat, spin around and knife the wall-mounted torch to reveal a crafty little Spinel.
El Gigante boss battle
Numerous goodies: Raid the three shacks and clear them out of all of valuables before the behemoth is able to destroy them. Apparently, in the GameCube version, once El Gigante obliterates a shack, all items stored within are lost, so it's wise to get in, grab what you can, and continue the fight afterwards. The PS2 and PC versions might not suffer this same problem, but I wouldn't recommend chancing it.
Yellow herb: Sitting on top of the wheelbarrow, next to a large collection of boulders and towards the back of the arena.
15,000: Kill El Gigante.
3,000 PTAS: Approach the alter which is situated opposite the entrance. On a cabinet to the left will be your pile of gold.
5,000 PTAS: Head back inside the church to find two crossbow-wielding Ganados standing at the altar. Kill them both; one will drop the gold.
A lot of gold: Shoot the cart which sits just at the top of the hill, in front of the church's gate, and watch as a few explosive drums soar off into the crowd, killing most with a monstrous explosion. It certainly saves on ammo, if nothing else.
1,100 PTAS: Shoot the bird's nest that's sitting on one of the branches in the tree next to the church's entrance.
Spinel: Upon leaving the cemetery area and setting foot in the narrow path which leads towards the underground cave, look upwards towards the tree branches to find yet another bird's nest which contains treasure.
1,000 PTAS: Not even a five second walk from the previous one; this next treasure can be found sitting atop a tree branch, inside a bird's nest, just ever so slightly further down the path.
Insignia house (revisited)
Spinel: Shoot the lantern which sits above the trapdoor to the cave. Someone seems to have fixed the last one that you destroyed in Chapter 1-3.
Spinel: At the very top of the bell tower, which is positioned next to the Insignia house that you'll have just exited from.
Numerous crates and items have respawned.
Numerous crates and items have respawned.
Yellow herb: On a side table, right next to the front door.
When given the choice of using the lever to open either the left or right passage, choose the left one.
Ruby: Kill the Bella Sisters; they'll the two chainsaw-swinging ladies with the bandaged faces. One of them will drop the ruby.
Red Gem: After using the Camp Key and gaining access to the next, really quite narrow area, dive inside the door immediately to your after after turning the first corner. Inside the room will be very little of interest, except for a boarded up window. Smash down the boards and hop through, then follow the passage along until you find two wooden barrels. Inside one of the barrels is your Red Gem.
El Gigante ambush path
In order to reach this area, you'll need to backtrack right the way up until the part with the Merchant, typewriter, and Luis' cabin; once you're there, use the lever to open up the right-side passage, sealing off the left-side passage that you've just completed, and head on through the door on the right.
15,000 PTAS: Kill El Gigante.
Lots of goodies: Search both shacks that turn up in front of you as you pass through the valley. Amongst the collectible items stored inside will be Spinels, PTAS, as well asammo and grenades. If you don't kill El Gigante, the shacks will eventually be destroyed and the treasures lost if you aren't quick enough to grab them beforehand.
Purple Gem: Hovering just above the second shack that you'll come across will be a bucket hanging from a wire. Shoot it to reveal the Purple Gem and have it fall to the ground.
Yellow Catseye: After riding the lift right the way across the canyon, head down to the first set of stairs and, instead of continuing down the next set of stairs, head across the wodden boards and into the cave on the other side. Follow the passage along and you will eventually come to a treasure chest containing the Yellow Catseye.
Chief Mendez boss battle
I'd recommend waiting until Mendez is defeated before bothering to collect all of the items from the barn.
Spinel: Destroy the red barrel that is situated near the ladder at the back of the barn.
Yellow herb: Climb the ladder and investigate the window ledge on the left.
30,000 PTAS: And, of course, your reward for killing Mendez. Collect it from his corpse.
5,500 PTAS: Destroy the incoming truck and investigate the wreckage. It's usually available for collection on the underside of the car, close to the rear tires.
Velvet Blue: Head up the hill, pull a left and smash open the tall wooden barrels to uncover the next treasure.