In this guide, I will explain which choices you need in the two minigames to take to get the best outcome for every girl. Here are some important notes before we start:

• Each girl has one very specific combination for both minigames, and picking the wrong one will make you fail.
  
• The cake minigame has a "start over" button, so it's okay to mess up, but the piano game is only one shot.
  
• If you're going to save at the piano minigame to try all combos, save before the minigame actually starts (save when Ermy is explaining the octaves the latest). Saving while in the minigame makes the game press 1 as soon as you load sometimes, so it's better not to risk it.

The cake minigame has a timer of 1 minute for you to make a choice - if you don't pick anything, a girl will pick her favorite choice instead, so don't stay still for too long.

Nana:

• Tart
  
• Cream
  
• Strawberry

Sima:

• Mille-Feuille
  
• Custard
  
• Cherries

Marta:

• Cake
  
• Chocolate Paste
  
• Peach

Any of these combinations will get you the achievement "Pastry Puffer".
Any other combination will result in a fail.

For this section, the number of the keys are in order (leftmost key is 1, rightmost key is 7). You can take your time with this minigame, as there isn't a 60 second timer this time around.

Nana: 7 1 5 3 4

Sima: 1 3 2 2 3

Marta: 2 2 1 1 5

Any of these combinations, in that exact order, will give you the achievement "Mensa Club".
Again, any alteration will result in a fail, but at least Ermy will like your music this time :P

• In the cake minigame, as mentioned, there is a timer for you to pick the ingredients. If the timer exceeds 60 seconds, one of the girls will pick her favorite instead. It's always Sima's base, Marta's filling and Nana's topping, so if you just idle for the entire minigame, the cake will end up as a Mille-Feuille filled with chocolate with strawberries on top. There is no special dialogue for this, you just fail the game.
  
• Also in the cake minigame, if you pick "Rum raisin" as the filling, you get a message saying "That's illegal!" and have to pick another filling. You can keep picking it, but it will just keep repeating the message, until the timer runs out and Marta picks chocolate.
  
• The other strange fillings (toothpaste, ketchup) or toppings (chocolate coins, strange emblem) do not give any special dialogue. You just fail the minigame.
  
• (This spoils the meaning of the choices) The minigames don't actually affect anything. They just give you the achievement if you do them right. So if you messed up in your first playthrough, don't worry, you don't lose the girl because of that.

I hope my guide helped you!