Highrise Level
You can reach a hidden balcony outside of the hotel on the Highrise level by climbing. To get there, you must jump from the planters to the electrical box to the second story ledge. Once on the ledge, you can jump to the balcony. From the opposite side of the hotel building, you can reach the roof of the adjacent building by running along the side edges. To do so, you must run across the middle ledge and jump to the electrical box. From the electrical box, you can climb to the roof.
Terminal Level
On the Terminal level you can get to the top of the plane. Once on the plane, you can run back and forth from either side, giving you a vantage point over other players. To do so, you must jump from the inside of the plane to the cargo boxes. From the cargo boxes, you can reach the top of the plane. However, you may fall while attempting to reach this secret area.
Estate Level
There are a few secret spots located inside and outside of the house on the Estate level. You can reach a support beam from the second story by jumping quickly between the hallway and the last room. Once on the support beam, moving to the far side gives you a view of the front door. Outside of the house, you can go underneath the boathouse by walking to the edge of the water and crawling underneath the support beams.
Warnings
While some secret spots contain items you can collect, most are used for hiding and vantage points. You can generally hide until another player finds you. However, if you choose to kill another player in online mode, the "Modern Warfare 2" kill cam will pinpoint your hiding place for other players. Once pinpointed, other players can easily find you.