It's hard to say, as the main game feels like a labyrinth when you've choose so many different choices, locations, or characters. The other 2 side stories are super easy with no issues on completing. The 1st story has 1 set of choices, the 2nd story has no choices, and the main story has the majority of trophies. Whereas, the guides are very limited since they are all for the PC "Hentai" version. I would use the g-seeker guide or the Chinese guide that was just added. Though, none of those guides are perfect, and the Japanese PS3 wiki site helped me find the last 2 wallpapers. If you have any issues with the CGs or Wallpapers, then I would recommend what TheNukes linked to the thread; it's awesome!
When the top-left box appears on the screen, it will state:
= "Congratulations! You have unlocked a new wallpaper."
At the end of the most routes, there will be new choices... I would highly recommend to create a save, so you don't have to redo the route again. Also, so you don't have to try to remember the route choices & locations. Though, I choose all choices that were available and they do matter to the wallpapers + CGs to unlock.
Edit: Got the platinum!!! ^^
There is also a mini-game for smashing a watermelon while being blindfolded. If you win, you get a hidden CG + Wallpaper. However, if you lack Japanese directions, then it can be difficult. There is no other mini-games unlike the PC version with the Skiing, Volleyball, etc. I would also highly recommend to make a save, before you choose the choice "ああ、わかった" in case you fail it. DO NOT make a save during the watermelon game, it's completely random and there is a time limit on each turn.
Japanese directions for the watermelon mini-game:
More Left = もっと左 (Motto Hidari)
Some Left = そのまま左 (Sonomama Hidari)
More Right = もっと右 (Motto Migi)
Some Right = そのまま右 (Sonomama Migi)
More Up = もっと前進 (Motto Zenshi)
Some Up = そのまま前進 (Sonomama Zenshi)
Down = 後ろ (Ushiro)
Close to the watermelon = スイカでしょう (Suika Deshouyo)
Hit the watermelon (Attack) = スイカよ！ (Suika yo!)
If you won, the watermelon will be smashed in half.
If you lost, the watermelon will remain with the stick.
This link has some details about the PC mini-games: