Several rewards which can be gotten for completing different in-game tasks.

Unlockable	How to Unlock
Armor/Dagger of Brutus	Complete all 6 Romulus Lairs and unlock the Romulus treasure room.
Bartolomeo's Axe	Complete all Mercenary Guild Challenges.
La Volpe's Bite	Complete all Thief Guild Challenges.
Maria's Dagger	Complete all Courtesan Guild Challenges.
Parachute	Destroy all of Leonardo's War Machines.
Raiden Outfit	Get at least Bronze medal in all VR Training Rooms Missions.
Sword of Altaïr	Complete all Assassin Guild Challenges.
War horses at stables	Renovate all stables in Rome.