There are four hidden houses in the game. Two of them are covered in Side Quests (Secret Lens Shop) and Seed Planting. The third house is located east of Lienea, in the forest. This is a programmer's room, where people will drop cryptic hints about the game. You can also find the unique Valkyrie item behind one of the pillars.
The last house is on an island north of Terazzi, in the forest. A man here enjoys playing Roshambo. There are no real prizes to speak of; it's just for fun. The basic idea is that you play an enhanced version of Rock-Paper-Scissors with Philia. The "enhanced" part comes from the fact that the winner doesn't win automatically. If you beat Philia, quickly press Circle to hit her. If you lose, quickly press Square to block her hit. You can play offensively and hit Circle every time you play, which is a gamble; you can play defensively and press Square every time, in which case you'll never win, or you can try to trust your reflexes, which is tough. 8-) It's good for a laugh, anyway.
Take Chelsea to the Neuestadt docks. First make sure you have two Bonitos (you can buy them at the fish monger nearby). Now talk to the cat at the top right. Feed it two Bonitos and it'll point you to a crate. Examine it for an Emerald Ring!
After getting the Horn to summon Bernardo, but before beating Belcrant, go to Armeida and talk to the sleeping woman in one of the houses. She'll give you 500 Lens. Make sure not to sell it! Go back to Darilsheid and enter the Oberon Corp. shop. Talk to the kid in the top right to get the Tricycle. The Tricycle is an accessory which takes effect when on the world map. It makes you move real slowly, but you won't be attacked by enemies when you ride it.
There are several accessories which let you fool around with how you play your battles. Let's take an in-depth look.
By default, Stahn can be on Auto (controlled by the computer), or Semi-Auto mode. In this mode, Stahn automatically targets an enemy (the target can be changed by pressing R1) and runs up to it when you press Circle. Skills are executed via the B button and a direction.
First up is the Tech Ring. Equipping this on Stahn lets him go into Manual mode (press Select). Now there's no automatic targeting; Stahn will strike wherever he is. To dash, press forward twice. This means that the strong dashing attacks are much harder to do, and it's a lot harder to hit enemies. This is by far the most challenging of Stahn's modes.
Next we have the Channeling, which lets a second character control someone. Give this to one of the other three characters and plug in a second controller. Now in battle, press Select on the second controller, and that character will be controlled by it! In fact, you can now turn Stahn onto full Auto mode and only play with the second character! This means switching controllers from 1 during field play to 2 during battles, but it's still fun. 8-) If you have a multitap, you can get three Channelings and three friends and have a blast!
Next up is the Combo Counter. Equipping this on Stahn means that instead of using the B button and a direction to execute skills, you have to do Street Fighter-like moves. To see a list of moves, go to Specials during battle and select Stahn. This is for fun more than anything else, because you'll never need access to more than four skills at once, and you can change his equipped skills during battle. Cool, though.
Finally we have the Dash Ring. This means that even when Stahn's on Semi-Auto mode, pressing forward twice will let you dash. This can actually be useful at times, but again it's more a novelty than anything else.
There are several points in the game where you have to make a decision which affects you strategically, sometimes immediately and sometimes later on. Here's a rundown.
1) You'll meet Uncle Bob in Harmentz early on in the game. Talk to him and he'll offer to give you some Gald. You have three choices. Accepting the gift will get you 10 gald, and refusing it will get you 10 Lens. Saying that it's not enough money will get you 50 Lens. However, if you choose this last one, you won't get a Holy Symbol from Uncle Bob much later on in the game.
2) In Moreau, after saving the kid from the soldiers, go talk to his mother, who'll also offer you a gift. Saying yes will give you a Charm Bottle; saying no to it will get you a far more useful Blue Talisman.
3) Before entering Heidelberg for the first time, you can go south to Cyril. Enter Mary's house here to see a scene. If you do this scene, Dalis will survive later on, but Mary won't be usable until you beat Belcrant. If you don't do it, Dalis will die, but you can get Mary as soon as you have Bernardo.
4) In Lienea, after meeting Philia, talk to the daughter of the village elder. If you tell her you and Philia are romantically involved, you'll get 15 Miracle Gels. If you tell her you really love her, she'll give you a Magic Mist, which increases escape speed by 50%. If you say you don't know, she'll give you the (most useful) Black Onyx, which increases max HP by 30%.
5) When Raiker asks you to get him an assistant, you have two choices. One is to get Otto's assistant in Darilsheid, and one is to get the man at the top right corner of Junkland. Once you fire the Multiplexed Lens Cannon, talk to the assistant to get an item. If you have Otto's assistant you'll get a Grow Fruit; if you have the Junkland one, you'll get a Laser Blade. If you didn't choose the Junkland one, you can buy the Laser Blade off of him in Junkland for 99,999 gald. Since you should have already gotten a Laser Blade in Helraios, Otto's man is the better choice.
When you get to the very last save point in Dycroft, be sure to save. Now restart the game and go into the sound test mode. A new option, "Talk", will have opened up. In the Japanese version, the characters could talk to each other on the world map. Since none of the sound clips were translated, though, this became pointless in the English version and so the feature was taken out. However, this new unlocked option lets you listen to the clips anyway. They're fun to listen to, actually. 8-)