You can complete the Key to Sector 5 side quest and reenter the Midgar slums anytime after Cloud rejoins your party after you’ve rescued Cloud from Mideel and gone through the Lifestream scenes. The quest involves digging up a key in Bone Village and traveling around the slums to pick up some items.
Those that have attempted to travel back to Midgar quickly realize that the gate back into Sector 5 is locked. The man standing outside of the gate mentions that he thinks he lost his key while he was on the excavation tour.
Travel to Bone Village up on the northern continent. Speak to the man and ask to “Start diggin’” for “Good treasure”. The Key to Sector 5 can be found near the smashed up plane on the main level of the dig site. Use the screenshot below to help guide you.
The Key to Sector 5 will be in the item box in the morning. Exit Bone Village and take the Highwind to Midgar - just southeast of Bone Village.
The first interesting place to travel to is the Church in the Slums where Cloud first met Aeris. The very first time you travel back to the Church you will see a quick flash of Aeris still tending the flowers in the Church.
She immediately disappears as soon as you approach. It is not truly known whether this is a game glitch or intended as there is no other reference to Aeris’ ghost in the game and the ghost cannot be seen again. The video below demonstrates Aeris’ appearance:
Despite Aeris’ ghost showing up in the Church in the Slums, there is no legitimate way to save Aeris, to prevent her from dying during the regular story or to continue the game with her in your party.
You can use cheat devices and/or debug the games coding to bring her back into your party but this will cause problems in cutscenes beyond the Temple of the Ancients where she was not meant to be included.
Make sure that Tifa and Cid are both in your party - you can swap your party members around at the 
Save Point in the Sector 5 Slum area.
Enter the Item Shop with the malfunctioning security gun and examine the computer to initiate the scene. You will receive Tifa’s Ultimate Weapon called 
Premium Heart.
While her Premium Heart is Tifa’s strongest weapon it unfortunately provides no Materia growth, so you may have to avoid using it other than for the toughest fights in the game.
You can also talk to the man in the Weapon Store who sold you the Batteries earlier in the game. He will try to sell you a unique item for 129,000 gil - the mystery item is 
Sneak Gloves.
It is a unique and rare item but it may not be worth the gil at this point unless you have a lot of extra (selling Mastered 
All Materia will help get you there eventually). Sneak Gloves increase your chances of successfully stealing items with 
Steal Materia.
Finally, you may want to go back and grab the Turtle’s Paradise flyer if you missed it before. Check out the Turtle’s Paradise side quest section for more information.
This is everything you can do back in Midgar as part of the Key to Sector 5 side quest.
