The seven wonders of Nibelheim quest takes place in Chapter 8 when you take to Nibelheim with Cloud and Sephiroth.
When you first arrive and have your cutscenes, you should see a boy to your right, he'll be the one giving you the tasks/clues. When you first talk to him, he'll mention that the area is home to seven wonders.
First Wonder: The boy will mention that the water supply to the town has been running red. This one should be obvious as to the location as you're standing next to it. Climb to the top of the water tower and remove the summon materia from the pool. Not only will removing the materia fulfill the first wonder, but you'll also receive the Phoenix Materia. Once you get down, go back to the boy and you can begin the second wonder.
Second Wonder: The boy isn't excited about your findings but he'll ask you to investigate a moving portrait in the inn, on the second floor. Once you investigate the portrait, go back and talk to the boy. Then you'll go back and see that the portrait has changed. Everytime you leave the second floor, the picture will change. Eventually, the manager behind the desk will make his way up the stairs and take off to the picture. Once he goes upstairs, follow him and make your way to the picture. When you approach him, you'll find out the secret to the second wonder. Following this, go back to the boy and talk about the third wonder.
Third Wonder: The next one can only be found by traveling around the town into the Outskirts and up to the Mt. Nibel Trail. According to the kid, there's an unusual bomb wandering around and it drops a rare item when it's beaten. To see what he's talking about, you'll follow the path to the east into the dead end. Once there, you'll face off against three bombs, who need to be beaten before they explode. Once downed, you should get a gold shard then go back to the boy
Fourth Wonder: The boy should talk about a laughing safe in the Shinra Mansion, meaning that you'll be heading there to find the safe code for its contents. Unfortunately, each save's combo is different. I had a tough time with this one just due to the obscured vision you're allotted. Once inside, the first digit you need can be found by peeking through the door on the eastern end of the second floor.
Inside, you'll count all the books that aren't shelved right. This means the ones on the floor, chair and on top of the bookcase.
Next, the second digit can be found through the eastern door on the first floor. Inside you'll have to count how many monsters are inside. Make sure to be patient when looking in the corners of the view as some drift a little out of view and barely come into frame.
Next, the third digit can be found on the top floor on the west side. In this room, you'll count the Banora Apples and the Canned Apple Juice in the room. Some may be on the floor, the nightstand and the dresser across from your view.
Finally, the last digit can be found on the west side room on the first floor. This room is quite simple, you'll just have to count the chairs in the room.
After all the digits are together, input them in the safe, grab the contents and go back to the boy.
Fifth Wonder: This wonder can be accessed once you return
*** Spoiler - click to reveal ***
. You'll be sent back to the Mansion with the clue of using the elevator that leads to the caves below. Once you're in the caves, you'll have to face off against some Sahagins and gain coffin keys upon beating them. Behind the two red doors, you'll find the coffins in question. Use the keys on the coffins. You'll know when you find the right one as a cutscene plays and the wrong ones will pin you against bizarre bugs. Return back to the boy, do not continue the main question, you won't be able to complete the Wonders if you do
Sixth Wonder: The boy will reveal this wonder as the "treasure within the flames" but he knows nothing else of it. To discover this wonder, you'll have to play through the story until you come across the boy again. His mother will be stuck in a burning building and its up to you to save her within a time limit. It might be hard to find the correct route but the best way to get to her it to walk to the right of the house, push forward to the back end, continue left up the stairs, come back towards the camera, and to your right. Once you find her, just do that backwards and you'll complete the wonder.
Seventh Wonder: The boy will try to explain the next wonder to Zack but he'll cut him off and delay it. To finish this side quest, you'll need to continue the story until you leave the Shinra Mansion in the next chapter. Upon walking away from the mansion, you'll get an email from the boy saying that he left a gift in the Outskirts. To find it, proceed to the northeastern path to the top of the Outskirts and then to the south. You'll see a glowing item behind a fence near the furthest house, this is what you're looking for. Pick this up and the Quest should end.
