The location of the Adamant needed for Excalibur is in the tower in the sky -- you need the chime and the cube. Go in the tower and continue to the place with eight ways to travel. Go in the bottom left diagonal and fight the enemy that guards it. Open the treasure chest to get the Adamant.
Find the three tombs located next to the white magic shop in Elfland. The tombstone farthest to the left reads:
"Here lies Erdrick"
This is a direct reference to a character in the game Dragon Warrior.
Northeast of the port town of Provaka is a two space wide peninsula with enemies from the Northern Continent. They offer a lot experience and are difficult to defeat unless you are around levels 12 or 13.
Acquire the ship from the pirates. Board the ship, and repeatedly press X + Circle fifty five times. A number-sliding game will appear. Complete this as fast as possible. If you finish under two minutes, you will receove 10,000 gil. Finish under four minutes for 5,000 gil, and under six minutes for 2,000 gil.
Go to Bahamut's cave. Once you first enter, go to the bottom left part. The Zeus Glove will appear in the items screen if you are lucky -- There is a 1/300 chance of finding it.
In FF2 board the snowcraft. Then press and hold (X). While holding (X) press (O) 20 - 25 times. You should get a game of concentration where you can win gil and various prizes.
After you get the Healing Staff, get in a battle with any weaker monsters. Kill all but one of them, then have someone use the staff and the others in your party use equipment that do not do anything. Continue until fully healed. This works well when going into the Ice Cave after the long river ride.
The battle system counts the amount of times you select an action, so to level up your weapon and magic skills quickly, press X twice to confirm the selection, then O to cancel. Repeat as desired. Note that you can only level up a weapon skill one time per battle, so don't waste 15 minutes trying to get 3 level upgrades in one battle.
In the first castle entered in the game, Castle Coneria, above Queen Jane's room is what appears to be a room with no one inside. Search around the room a bit, however, and you will realize that you can talk to someone although they are not there. After you rescue the princess, the message will change.
Note: Corneria was changed to Cornelia in later versions of the game.
First, your characters should be strong enough to withstand an attack from either Maria or Mindu in the earlier part of the game, and Leila or Gordon in the later part. I suggest 150+ HP for Guy, 50+ HP for Maria, and 85+ HP for Firion before you try this.
Run into a battle near the Altair region. The monsters are insanely weak compared to you, so no harm can be done from them.
Have either Maria, Mindu, Gordon, or Leila attack your party members while your party members cast spells like Shield, Shell, Blink, or any spell that doesn't damage the enemies. Watch out in using two weapons--there's a chance that one of the characters might die from your own attack.
Get them down to about 1/4 their remaining HP. Feel free to also cast magic on yourself. Your HP will go up rapidly. I got Leila from 150 HP to 750 HP in about 15 minutes, and Gordon from 68 to 511 in about 10 minutes.
This helps increase your magic strength and weapon proficiency as well.
Oh yeah, FF2 only, in case you didn't know.