First floor:
* Fluffy: A three-headed dog.
Second floor:
* Flying keys: Keys that flew around the room, trying to attack anyone who entered.
Third floor:
* Troll: A monstrous creature guarding a locked door.
Fourth floor:
* Chessboard: A giant chessboard where players had to battle each other.
Fifth floor:
* Devil's snare: A deadly plant that traps its victims.
Sixth floor:
* Potions: A variety of dangerous potions that had to be chosen carefully.
Seventh floor:
* Mirror of Erised: A magical mirror that showed the deepest desire of one's heart.
The final obstacle wasn't specifically guarding the stone itself, but rather the final hurdle that needed to be overcome to reach the stone.
It's important to remember that these obstacles were set up by Professor Quirrell (under the control of Lord Voldemort) to prevent anyone from reaching the stone.