Key to Sector 5 & Tifa's Ultimate Weapon - Disc 2
On Disc 2 once you get Cloud back from his comatose state in Mideel you'll be able to enter Midgar again and pick up some new items. Also, this is an ideal time to do Turtle's Paradise Flyer No 1 if you missed it the first time you did Midgar.
To get back to Midgar you'll first need to go to Bone Village and dig up the Key to Sector 5. You'll find this item by having someone dig near the smashed up plane just across from where the lead excavator guy stands. It's the same location you see me standing in the screenshot above.
Inside Midgar there are two treasures that you can grab, one requires you to have at least 129,000 Gil and the other has no requirements. If you don't have the Gil don't worry, it's still worth coming here because Tifa's Ultimate Weapon is the other item.
You'll find both items in the Wall Market location, the 129,000 Gil item is sold by the same old guy that sold us the Batteries earlier in the game. This item is called Sneak Ring and it increases your chances of successfully stealing an item by quite a bit.
If you've forgotten where this old guy is, he's at the northern end of the Wall Market, the building with all the machine parts out front and the large neon sign that says Machine Gun.
As for Tifa's Premium Heart Ultimate Weapon, you'll find that in the shop marked "Item" in the entrance area of the Wall Market. Previously this shop had a machine gun in it that fired at you if you messed with the computer - that's broken now and you can collect Premium Heart.
Despite this being Tifa's Ultimate Weapon, it has no Materia Growth which means it basically sucks unless you don't have any need to level your Materia anymore. It's not entirely useless, you can equip it on her before big boss fights or once you've mastered all the Materia you wish to master. Still though, it's disappointing, I know.