It may be more of a poorly maintained dirt road than true off roading, but if you head out to polihale state park there's some terrain you need a jeep to get past and you can also drive on the beach.
Then if you're over by poipu beach and head east past the Grand Hyatt and keep going onto a dirt road and through a few fences the road ends at a beach that's out of the way and isn't too crowded.
Have fun!