CPR CREW BECOMES COMPLACENT WHILE SIGHT-SEEING DURING APCH TO CRQ; CA; UNTIL ATC QUESTIONS WHERE THEY ARE GOING.

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WHILE ON AN IFR FLT PLAN FROM SMO-CRQ; WE DEVIATED 40 DEGS FROM OUR ASSIGNED HDG. WE WERE IN THE DSCNT PHASE ABOUT 20 MI FROM CRQ. IT WAS THE FO'S LEG AND I WAS ADMIRING THE SCENERY ON A BEAUTIFUL CAVU MORNING. WE HAD BEEN GIVEN SEVERAL LARGE VECTORS DUE TO TFC. AS WE GOT CLOSER TO CRQ; WE GOT ASSIGNED A L TURN OF ABOUT 30 DEGS. AT THIS TIME; WE WERE GIVEN OUR EXPECTED RWY ASSIGNMENT. I MOVED THE HDG BUG TO THE ASSIGNED HDG; THEN I PROCEEDED TO PROGRAM THE FMS FOR THE APPROPRIATE RWY. WHEN I LOOKED DOWN TO WORK ON THE FMS; I DID NOT REALIZE THE FO WAS STILL LOOKING FOR A PARTICULAR PIECE OF SCENERY THAT I HAD TRIED TO POINT OUR EARLIER. HE CONTINUED PAST OUR ASSIGNED HDG ABOUT 40 DEGS. WHEN I LOOKED UP FROM THE FMS; I CALLED THE DEV TO HIS ATTN. HE IMMEDIATELY EXECUTED A 45 DEG BANK TURN BACK TO THE PROPER HDG. THE CTLR QUERIED US AT THIS TIME AND WE TOLD HIM WE WERE CORRECTING. THERE WAS NO OTHER ACFT IN THE IMMEDIATE AREA. I BELIEVE THIS DEV WAS CAUSED BY A LACK OF COM BTWN THE FO AND MYSELF AND A PREOCCUPATION WITH FMS PROGRAMMING THAT WASN'T ENTIRELY NECESSARY. I SHOULD HAVE LET MY FO KNOW THAT I WAS 'HEAD DOWN' AND I SHOULD HAVE NOT FOCUSED ALL OF MY ATTN ON THE FMS. I HAVE LEARNED THAT ESPECIALLY WHEN IN UNFAMILIAR PLACES IT IS IMPERATIVE TO LEAVE ALL SIGHTSEEING TO THE PAX UNTIL ALL OTHER DUTIES ARE DONE. DOING TOO MANY 'HEADS DOWN' THINGS CLOSE TO THE ARPT MAKES US MUCH MORE SUSCEPTIBLE TO STUPID MISTAKES; ESPECIALLY IN GREAT WX.