Yes, you can drop off a car in Whittier, Avis and two other companies are within walking distance of the cruise ship terminal (e.g. https://alaskaautorental.com/). Typically one way drop off charges are pretty expensive in Alaska but you could also return the car to where you rent it in Anchorage and take the bus or train to Whittier. Start at https://www.alaska.org/advice/anchorage-to-whittier to check schedules and prices.