* On Farm Road level, go under the train trestle in the middle of the pond to
  find $1500. Note: This works only one time.

* On the Restricted Area track, go straight when you pass through gate 5. After
  a little driving you will reach an aircraft hanger with a jet inside. In the 
  corner of the hanger is a chest of cash.

* On the Laguna Del Ray track, off on the right is a small island just offshore.
  On top the island is a chest of cash.

* Between gate 6 and 7 on the Black Gold track is a fenced-in area that contains
  another box of cash. There is an oil truck nearby it.

* When starting on the Treasure Bay track, go over to the beached tanker boat 
  to the right, and drive around on the water side. You can find a chest in the
  side hull, with $666.

* On the Truck Stop 101 track, go to the "hidden" highway and drive alongside
  the right side of the road. You will eventually find a chest of cash.