Spring Cave: NW corner (Guided Warp) / NE corner (Heart Slippers) / SE corner (Gold Bug)
Casino Cave: NE corner (Crystal Choker) / SW corner (Gold Shoes) / NE of center (Sleep Bow)
Lava Cave: B2F, SW area (Fire Bracelet) / B3F, SW area (Shinobi Shield) / B3F, NW area, Hidden (Dream Rod) / B3F, NE area (Ring of Magic)
Lost Forest: N area, Hidden (Silver Sword) / SE of center (Gold Bug) / W area (Warrior Shield)
Daunting Mountain: 1F, SE area (Ring of Speed) / 2F, SE area (Flash Bomb) / 2F, NW area (Skeleton Key) / 3F, N area, Hidden (Mirror Shield) / Clouds, N area (Aura Knuckle)
Sunken Shrine: B2F, NE corner (Ring of Power)
Pyramid: 2F, NE corner (Golden Shield)
Underground Passage: NW corner (Charm Potion) / E edge (Nice Guy Badge) / NE area (Elven Bow) / NE area (Moon Shield)
Tower of Rabble: 1F, SE area (Ring of Defense) / 1F, SW area (Ring of Magic) / 2F, N area (Toy Hammer) / 2F, NE area (Dokapon Orb) / 3F, NW area (Soul Eater) / 3F, NE area (Cursed Shield)
Underworld: 1F, center area, Hidden (Dokapon Crown) / 1F, S area, Hidden (Athena's Shield) / 1F, SE area, Hidden (Great Tempest) / 3F, N deadend, Hidden (Punisher) / 3F, NE area (Dokapon Shield) / 3F, NW area (Dokapon Sword)
As far as the accessories in this list are concerned:
Heart Slippers: +3 SP, heals 1 HP per space traveled.
Crystal Choker: Similar to Robo-Knight's innate, small chance each turn of a free Multi Crystal use. Roughly 20% if I had to guess.
Gold Shoes: +3 SP, gain X gold per space traveled. X is roughly equal to 90.
Fire Bracelet: +3 MG, halves damage from fire field magic. Any fire-based status effect field spells will have a 0% hit rate (if there are any. I know Wind Bracelet makes Squall have a 0% hit rate)
Ring of X: +15 to X.
Nice Guy Badge: +3 AT. Apparently reduces the time you are Wanted if you fail to rob a shop/town. Normally you are Wanted for one week. With this badge, you are only Wanted for half a week (3 days of not being able to land on buildings). --- Less than a week, and "All Day Assault Farming" has taken over. Lunar2/Xuande
There is more than one locked box on the second floor of the Lava Cave (three, I think: one for each separate area of the floor). I seem to remember that one of them contains a Reveal.
Also, there are two more locked boxes in the Tower of Rabble. The first one is on the second floor at the eastern edge; it contains a Dark Sword. The other one is on the third floor, in the northwest corner at the end of a short hallway; it contains a Sun Shield.
In one of my first playthroughs of the game, I experienced something strange regarding the hidden locked box in the Lost Forest. Since I didn't know about invisible spaces at the time, the box was ignored throughout much of the game. Much later, in Chapter 6, an AI player who was down on his luck just happened to be there with a magic key. He opened the box (surprising me in the process) and got... a silver SHIELD. Could this mean that some or all of the locked boxes can give out different treasures depending on the circumstances at the time they were opened? (I've never seen anything like this happen ever again.)
I am absolutely sure that it was not the warrior shield. I remembered this incident partly because it was the first time I had seen an invisible space. (Come to think of it, it may have been the first time I had seen someone open a locked box. Wow, the equipment used in that game must have been REALLY boring if that's true.)