You need to do and collect everything:
- Finish all 15 story levels achieving True Avenger.
- Collect all 150 minikits in the story levels.
- Collect all character tokens (story levels and hub worlds), then buy the characters.
- Collect all vehicle tokens (story levels and hub worlds), then buy the vehicles.
- Collect and buy all 18 red bricks.
- Collect all 250 gold bricks.
You need to save all Stan Lees in order to receive the character token Stan Lee.
You can check your completion percentage in the pause menu. When you reach 100%, fly to the top of Avengers Tower in Manhattan, enter the building, and take the elevator down. Flip the switch on the chest surrounded by furniture to activate the stud fountain.
I was stuck at 99.9%. Turns out, the vehicle stations in Central Park offer different vehicles than the vehicle stations anywhere else in Manhattan. So make sure you buy all vehicles in Central Park and Manhattan, as well as in the other hub worlds.