When you first find the AIRSHIP, you won't have an immediate destination among the new areas you now have access to. The most logical starting point is Gaia, a town northeast of the Yahnikurm Desert found on the northeastern continent. This is the only town you can access in the north without trekking across dangerous terrain, and there is a variety of nice new gear to buy here.
Gaia
The most valuable thing to find in the sprawling town of Gaia is information. This is where you will learn that the fairy that used to live here was put in a BOTTLE and sold to a Caravan. You will also hear a few hints about the language of Lefein to the south. While you're here, don't miss out on the various upgrades that are available. Things are quite expensive here, but you can find a lot of money by visiting the various caves of Cardia if you need it. Keep in mind that you will need 50,000 G in the near future to buy the BOTTLE you were told about.
New Equipment
There is only one weapon available in Gaia, the obscenely expensive CatClaw. No one can use it until after the Class Change, and even then it's only really useful to a Bl.Wiz. Aside from the Masmune itself, the CatClaw is the most powerful weapon a Bl.Wiz can use. That said, you will soon start finding a lot of items that cast spells in battle for free. Your Bl.Wiz will be rarely, if ever, attacking with a weapon. Save the 65,000 G until you have more money than you know what to do with.
The armor available in Gaia is much more interesting at this point. The Gold Bracelet is the best Bracelet you can buy, and represents the best armor that can be purchased for every class except the FIGHTER. (The NINJA can also use better armor after the Class Change.) However, Gold Bracelets are extremely expensive at 50,000 G each, and you will find one for free shortly in the Castle of Ordeal. Buy them if you can spare the money, but don't blow your savings outfitting everyone in the party if you can't afford to.
The ProRing is available for the much more reasonable price of 20,000 G, and is the best piece of hand armor for every class in the game, KNIGHT included. In addition to providing the best available Absorb with a minimal Evade penalty, they offer resistance to Death. Note that this doesn't make the wearer immune to all KO attacks, since many are Earth- or Poison-elemental, or even non-elemental (and thus not resistible). Still, it's a nice resistance to go along with the 7 Absorb upgrade over the Gloves that mages and Bl.BELTS are likely still using. If you can't afford everything available here, ProRings are arguably the best thing to spend your money on. That said, you will find three for free much later in the game.
Spell Selection
Gaia features both level 7 and level 8 magic, but in both cases you'll find only a limited selection of the available spells at this level. The other level 7 spells are for sale in Onrac, and the missing level 8 spells are available in Lefein. Prior to the Class Change, the only spell any class can learn here is ICE3. If your Bl.MAGE has reached level 20, this is a worthwhile investment.
After the class change, a variety of new spells open up to your wizards. RedWizzes don't get much at level 7—the only spells they can learn at all are ICE3 and ARUB, and of the two only ICE3 is sold here. They will eventually gain level 8 spell slots, but there are no level 8 spells that can be learned by a RedWiz.
Bl.Wizzes also have an easy choice. The SABR spell sold in Onrac is bugged and does not function, so you may as well pick up BRAK here if you can spare the 45,000 G. It's an accurate, single-target, Poison-elemental KO spell that may even see some use, but it's nothing special.
Wh.Wizzes will definitely want CUR4. Unlike previous CURE spells, there is no comparison between CUR4 and the HEL3 spell available in Onrac. CUR4 heals a Warrior fully and removes every status ailment save Stone and KO. HRM4 is not nearly as good—not because it is ineffective, but because there just aren't enough powerful Undead left in the game to justify using it, especially not over CUR4. Both spells available in Onrac (HEL3 and ARUB) are decent, so feel free to give HRM4 a pass.
Once your Wh.Wiz or Bl.Wiz reaches level 25, they can cast level 8 spells. Three of each type are sold here, but the missing fourth spells (available in Lefein) are the best in the game, so make sure to leave room for them. Wh.Wizzes have it easy: XFER doesn't work (at least not when you cast it; it works just fine for monsters), so you may as well pick up WALL and FADE. WALL gives one Warrior the same resistances as a Ribbon, which is helpful since there are only three Ribbons in the game. FADE is actually Holy, and is a nice non-elemental attack spell. You probably won't want to waste potential LIF2 castings with FADE, but it's still fun to have.
Every level 8 Bl.Wiz works, but there are some weird choices here. ZAP! and STOP are similar spells: both are Time-elemental, accurate, and hit all foes. ZAP! causes KO, and is only slightly less accurate than STOP, which only cuases Paralysis. So why would you cast STOP? Beats me! XXXX isn't too impressive either. It is a KO spell that will kill any monster with less than 300 HP unless they resist Death. The problem is, by this point 300 HP isn't a big deal to cut through. Looked at another way, XXXX is, at best, a spell that deals 300 damage to a single target. NUKE is almost always a better use of a spell slot.