Cathy,
Do you remember any names of nearby towns, what road you were on, or any landmarks?
Let's see if I can jog your memory; I'll throw some names out that are nearby where redwood trees and sand dunes are found in the same location or nearby each other, where you could camp.
Starting from the North and working South in Northern California...
Smith River
Crescent City
Mill Creek
Klamath River
Fern Canyon
Praire Creek
Orick
Redwood Creek
Freshwater Lagoon
Dry Lagoon
Stone Lagoon
Big Lagoon
Patrick's Point
Trinidad
Luffenholtz Beach
Moonstone Beach
Little River
Clam Beach
Mad River
Arcata Bottoms
Eureka
South of here...
Elk River
Cock Robin Island
Eel River
Ferndale
Wildcat
Petrolia
Mattole River
Lost Coast
Shelter Cove
Fort Bragg
Any Of These Names Ring A Bell?