The easiest option is park up but its $60 for parking ontop of 4 days rental sat doing nothing.
Another option is rent thru Budget, no charge to drop off at their port location with free shuttle to the ship. This saves 3 days rental as when you collect after the ship your rental starts early morning but drop off at airport after this time so only saving 3 days not 4 days rental. That would have saved us about £52 last month ontop of the £30 parking. Another bonus is the car you collect after your cruise includes another tank of gas(if you take gold cover, we always do) so about another £30 better off.
All in we would have been better off by about £112 dropping the car, this time we didn't but next year is a 7 night cruise so the saving is nearer £200, although we like having the car sat at the port for storage I just can't pay £200 for it.
It can be easily done thru USrentacar, they use budget and offer port location.