Here is a long list of available cheats in Final Fantasy II.
1. Here we go, the ultimate FF2 cheat! This is a way to turn your characters into walking gods in about an hour. You can do any variation of this cheat, but here's my preference. First and foremost, buy a "Cure" spell for the 3 main characters, as the 4th character you get could leave the party at any time. Second, make sure you have about 500+ gil (it all depends on how powerful you are). Now, get into a battle against real weak enemies (Goblins, Leg Eaters, Hornets), and start pounding on your own guys. Keep on pelting your guys with damage, until they're all in their "low HP" position. Save your game after every round, if you're using an emulator, just in case somebody dies. Win the battle, and your guys should gain some HP, and possibly Strength points or Defense points. Enter another battle, and constantly cast "Cure" magic on your guys, who should have low HP. Once all their HP is full, or all their MP is used up, win the battle, and you'll gain more MP. I've noticed that the amount your Max HP/MP goes up increases when your Max HP/MP gets very high. My Frioniel has 4000 HP, and every time his HP goes up, it increases 80+ points! Finally, after completing these steps, go to an Inn, rest up, then repeat as necessary. It's helpful to be at extremely high HP, due to the complexities of the game's dungeons. Don't power up that many spells, only Cure, Fire, Ice, and Bolt, or whatever spells you use constantly. Weapon points go up as you kill enemies, and when they reach 100, the character becomes more adept at using his/her weapon, allowing for greater hits. Build up the weapon levels while fighting the strongest possible enemies you can at whatever stage you're on.
2. If you're playing FF2 through an emulator (which is very likely, due to the rarity of the actual cart), you probably know that the number of steps it takes to get into an encounter is determined as soon as you enter the level. The same holds true with FF1 and 3, but strangely, not with the Dragonquest games. In case you're extremely lazy, are using an emulator with a save state function, and don't want to get into a battle, do this trick. Walk, and after every step, save the game. Keep doing this until you get attacked. Load the save game RIGHT before you get attacked. Now, go to the Status Screen and cast the Cure spell on one of your party members. Exit the Status Screen, save your game again, then continue walking, until you get attacked again. Then, repeat. Unfortunately, for this trick to work, one of your characters needs to have gotten damaged, with HP lower than the max.
3. Like in FF1, you are able to view the complete world map. When you're on the world map screen, hold "B", and press "Select". This will bring up a screen with a giant globe in the middle of it. Press a button on the direction pad to rotate the globe in that direction, but be wary, as the rotation proceeds extremely slowly. The flashing brackets indicate the party's current position, and any flashing dots indicate important locales, such as towns, caverns, or castles. Note that this trick only works on the world map, and AFTER you've gotten the Ship (although you don't need to be on the Ship to see the map).
4. The Chocobo is directly south of the Kashuon Castle. From the Castle, just hold "down" on the controller, and you'll get to the Chocobo's forest. Catch it (it's TRES fast!), and ride around!
5. Brian Pacula told me a real cool trick to getting lots of gil. First, you'll need to be slightly strong, with over 500 HP. Now, head a little ways north of Phin. Around here lurk Magicians, and if you're unlucky, Land Turtles. The reason why you need lots of HP is in case you run into these asses. Magicians, when defeated, sometimes drop spell books. The spells aren't all that useful, so sell them for cash. You'll gain a lot of cash this way, and be able to buy powerful armor and weapons.
6. In the village of Mysidia, you can buy many types of Magic, both White and Black. This includes the powerful Holy spell, sold for a whopping 20,000 gil! You're also able to purchase the Ogre Axe, which is an extremely strong axe weapon. With this, and Guy with a weapon level of 6, I was doing 700 damage every time! Even before you retake Phin, and it's required to travel to Mysidia, you can go here and purchase these items. As soon as you get the Ship, just head to Mysidia, southwest of Altea, and purchase away! Be careful, as the enemies around here are STRONG! If you use the "skip an encounter" trick, you'll make it here alright. These items will help you greatly as you go to the Cave of Dist, and advance through the game.
7. There is a way to stock up on Magic Levels extremely easily, but it requires patience. Get into a battle with some enemies, preferably weaker ones, to begin. When the person with the spell you want to use comes up (it has to be one of the 3 lead characters for this trick to work), cast the spell, target the enemy, and all. Press "B" to cancel the last character's move, then recast the spell on the enemy. Repeat as necessary. Every time you do this, your Spell Points will increase by 1. Do this 100 times, and you'll gain another Spell Level. Unfortunately, you can only gain 1 Spell Level per battle, and it takes an awful amount of time to do this trick, but it's worth it!
8. There's also a way to stock up on Weapon Levels. It's almost exactly like the Magic Level trick. Get into a battle with some enemies, then have the person you want to use Fight. Cancel that command, and have him use Fight again. Cancel, then Fight, then Cancel, then Fight, etc. for 100 times, and you'll gain another Weapon Level. Unfortunately, you can only gain 1 Weapon Level per battle, and it takes an awful amount of time to do this trick, but it's worth it!
9. There's a secret spell shop in Jade Caverns, where you can buy the spells of Death, Berserk, Haste, and Flare, for a large price. Once you get to the bottom of the HUGE waterfall in Jade, when you're wading in the water, walk underneath the waterfall. In the middle of it, you'll hit something. Search it, and you'll find the shop.
10. If you think Cid's Airship goes too slow, there's a way to give it an extra speed boost. Rapidly press the "B" button real fast 40 times. Then, you'll start to go really fast. Hold down "B" and you can keep going at this speed. It makes trips around the world A LOT easier!