Some things you might have missed is enemies that only appear in 1 shot places. There is a bomb on the ghost train, Ingari (Sp?) on triangle island, as well as some others listed in the world of ruin. The gameboy version is best to find all enemies cause there is a bestiary in the menu. Fighting every enemy is good only if you want to take the ultimate challenge of the game. Rages.

Some important items you can get require you to complete ever side quest available. There is a man sleeping in a bed where you have to send letters and check on him to get a really good relic.

A lot of the earlier chests in the game will hold different items if you skip them the first time and come back later in the storyline. Mostly noted the ones up until South Figaro. This is how you can get a ribbon in the World of Balance early on.

Strago has a lore ability where someone who is not using him regularly would miss. The best can be found from bosses in the final dungeon.

If you like treasure hunting them you should get the Ragnorak esper rather then the sword, but the sword is the best one in the game next to Atma Weapon. With the esper though you will be able to turn enemies into items in battle.

A good leveling up area would be a little north of the Veldt and east where a forest in the shape of a t-rex head is. On lower levels you should equip the Exp egg on and walk around the forest with three characters stoned. Have the proper esper equip and watch your stats fly. When you get to mid levels you should try out the forest itself as that is the best leveling up spot in the game.

All enemies in the game can be found on the veldt bar some bosses. This will allow a great day with Locke and the ragnorak esper. It is also a place where you can learn spells with no level gains. This makes sure you save your levels for stat boost and have the greatest spells learned fast. The other great places to gain AP is the tower where you find Stargo, or the desert where you can fight cataurs.

Sorry I can't add all the information, it's been a while since I played and have been focused on other games.