Aeris comes with the “Amplify” materia and an exclusive accessory: Cetra Ring. Yuffie and Cid come with new materias: Exp UP (Cid) and Gil UP (Yuffie). I recommend going for Yuffie as soon as you reach the forest area on the outskirts of Junon.
Personally, I think AP Plus is one of the best materias in the game. It’s not hidden; it’s in Junon, where the second “Enemy Skill” used to be. By the way, in Wutai you can buy as many “Enemy Skill” materias as you want.
In this cave you’ll find “Added Morph,” but it’s not missable — you can also find it in the Sunken Gelnika.

FORT CONDOR Rewards in order:
1- M.Comb
2- Peace Ring
3- Silver Glasses
4- Titan Guard
5- Mind Source
6- Mind Source
7- Super Ball
8- Turbo Ether
9- Turbo Ether
10- Speed Source
11- Speed Source
12- Speed Source
13- Elixir x3
Regarding Aeris:
You play with her throughout the entire first disc. After the events in the Forgotten Capital, she will separate from the group, but she will return in Disc 3. You can fight Emerald Weapon and Ruby Weapon with Aeris in your party. However, she will not descend into the crater to fight.


Aeris has an exclusive sidequest. Go with her to this house and, of course, save before opening the chest. There you will find her ultimate weapon and her Level 4 Limit Break.

Cloud has an exclusive accessory: Ribbon of Memento, a gift from his mother. You can obtain the accessory after Cloud rejoins the group in Disc 2, following the events in the Lifestream with Tifa. The NPC even has new dialogue.

These would be some of the new features. I recommend using the Steal materia against bosses, as in some cases there are changes.
I hope you enjoy the experience.