Defeating Electro
When you fight Electro, about half way through the game, hit him with your tendril. When he turns blue, use the stylus to throw blue mail boxes and the car. The car will respawn if you destroy it. If he gains health, use the tendril when he is red to take out the street lights. The lights will flicker after you hit them, but if they are not out he can use them to regain health.

Play as Carnage
Get a 80% game completion to unlock Carnage in Multiplayer mode.

Play as Symbiote Spider-Man
Get a 100% game completion to unlock Symbiote Spider-Man (black costume) in Multiplayer mode.