There is a whole lot to do in New San Antonio, and all of it is important to completing the game.
Your first task here will probably be to raise your Stats. You need a bare minimum of 29 Strength (to Wear Armour that will allow you to survive in space) to complete the game. Stat increases cost 100 Gold per 4 points, but things aren't quite that simple. The Game Systems page has a detailed breakdown, but the gist is that the Stat that is increased is randomly determined. Note that offering more than 100 Gold will give you a larger boost (up to +36 for 900 Gold), which can save time when saving and reloading looking for that one Stat you need most.
New San Antonio is also the easiest place to obtain a Plane. Planes are useful because they allow you to go almost anywhere and, unlike Frigates, can be taken to other eras through Time Doors. You'll need three Keys (you get two for each Guard you kill), a Brass Button, and a Skull Key to take the Plane. The Fighter in the air port will attack, but you shouldn't have any issues with Guards. Simply Board and Launch the Plane, and it appears on the world map.
After you've been to Planet X and received Father Antos's blessing, use two Keys and speak to the Merchant under the "atree" sign. This is the "old man under a tree" that is mentioned by the seer's Hint poem. Offer him 500 or more Gold to get the Ring. You can't get more than one, but if it should be stolen by a Thief, you can return here and buy another. (You don't need to re-obtain Father Antos's blessing.)
Getting the Quick Sword is tricky, though you can do it as soon as you have two Keys, if you're tough enough. It's the best Weapon in the game, but it only deals 2 more damage per hit than the Phaser and requires 49 Agility to wield. Contrary to popular belief, you do not need the Quick Sword to complete the game. (Which makes sense, since there's really no indication anywhere of how to get it.)
If you want it anyway, Unlock the prison and defeat the Guard inside. Then Unlock the middle cell on the left and Offer Santre the Swashbuckler at least 500 Gold. He'll give you the Quick Sword. That's the easy part—the hard part is cutting your way through the Guards to get out alive.
There is a cheap method to escape the prison if you can't handle the Guards. First, Unlock the prison, but don't attack the Guard inside. Follow that by stealing a Plane, attacking the Fighter in the air port. Fly out and land near the prison, drawing the Guard out and away from the door. Once he's far enough, Launch the Plane again and Land closer to the prison, leaving the Plane blocking the doorway. Get the Quick Sword, then return to the Plane and Launch it once more and fly out of town. Remember that you need five Keys to do this, as well as a Brass Button and a Skull Key.
Your first task here will probably be to raise your Stats. You need a bare minimum of 29 Strength (to Wear Armour that will allow you to survive in space) to complete the game. Stat increases cost 100 Gold per 4 points, but things aren't quite that simple. The Game Systems page has a detailed breakdown, but the gist is that the Stat that is increased is randomly determined. Note that offering more than 100 Gold will give you a larger boost (up to +36 for 900 Gold), which can save time when saving and reloading looking for that one Stat you need most.
New San Antonio is also the easiest place to obtain a Plane. Planes are useful because they allow you to go almost anywhere and, unlike Frigates, can be taken to other eras through Time Doors. You'll need three Keys (you get two for each Guard you kill), a Brass Button, and a Skull Key to take the Plane. The Fighter in the air port will attack, but you shouldn't have any issues with Guards. Simply Board and Launch the Plane, and it appears on the world map.
After you've been to Planet X and received Father Antos's blessing, use two Keys and speak to the Merchant under the "atree" sign. This is the "old man under a tree" that is mentioned by the seer's Hint poem. Offer him 500 or more Gold to get the Ring. You can't get more than one, but if it should be stolen by a Thief, you can return here and buy another. (You don't need to re-obtain Father Antos's blessing.)
Getting the Quick Sword is tricky, though you can do it as soon as you have two Keys, if you're tough enough. It's the best Weapon in the game, but it only deals 2 more damage per hit than the Phaser and requires 49 Agility to wield. Contrary to popular belief, you do not need the Quick Sword to complete the game. (Which makes sense, since there's really no indication anywhere of how to get it.)
If you want it anyway, Unlock the prison and defeat the Guard inside. Then Unlock the middle cell on the left and Offer Santre the Swashbuckler at least 500 Gold. He'll give you the Quick Sword. That's the easy part—the hard part is cutting your way through the Guards to get out alive.
There is a cheap method to escape the prison if you can't handle the Guards. First, Unlock the prison, but don't attack the Guard inside. Follow that by stealing a Plane, attacking the Fighter in the air port. Fly out and land near the prison, drawing the Guard out and away from the door. Once he's far enough, Launch the Plane again and Land closer to the prison, leaving the Plane blocking the doorway. Get the Quick Sword, then return to the Plane and Launch it once more and fly out of town. Remember that you need five Keys to do this, as well as a Brass Button and a Skull Key.