For strictly overnight parking, I've succeeded or gotten lucky a few times backing the car with rack attached into a holly bush or similar prickly landscaping. A couple or three locks (one U-lock, one keyed chain, and one cable with combination locks) have been enough to secure 1-2 bikes to the rack for meals and what I'd call light tourism (an hour or two in a museum, for instance).

But when you're there and headed to work, definitely keep the bikes either in the car or in a motel room.