The Monserrate is a peak overlooking Bogota. You can either walk up or catch the cable car. It is a breathtaking view of the whole city and the mountins on the other side. And is a great place at both day and night (or sunset and get the most of both).
It is a bit dangerous walking up, so the cable car is a better idea. And avoid the weekends if you are taking the cable car since it is packed!