Have done this one many times, 16 to 18 days R/T Vancouver and found the 5 days back were very relaxing, a time to decompress after 5 or 6 days straight in Hawaii. The two Hawaii cruises we did this year were a bit different because those days back did not have quite as much going on, few fewer classes, etc, but still lots to do if you that way inclined. Food on these cruises did not include a lot of repeats (as Alaska does, week after week!) Agree with Crystalspin, you should be fine. I would suggest you make reservations for any specialty restaurants early on because a lot of pax use those final days to "dine out" and reservations can sometimes be hard to come by.
Enjoy your cruise!