Rather short notice, but Alaska might be worthwhile.

Have they been to the Rockies or the west coast? Fly to Calgary, get a car, do a loop - Calgary - Banff/Jasper - Vancouver - Seattle - Yellowstone/Glacier - Calgary. GREAT itinerary with teens.

Any European family heritage? You might be surprised that the kids would respond positively to a little family tree climbing. Since time is short, get them involved with the pre-trip research.