Have sex on the balcony. The fresh air and thrill of the possibility of being noticed will make the experience vivid, though perhaps short.
For this practice it is better to choose the balcony of your own apartment, but if you are the adventurous type you can use the fire escape balcony. A safer option would be to spread a blanket on the floor and have sex while hidden by the guard railing. If you want to make the most of this experience, choose standing positions.
Be careful if the balcony is not glassed-in! Take care of yourself and observe the safety precautions so that the prying eyes of your neighbors will be the least of your worries.