I think your parents might really enjoy taking the Horse Drawn Trolley tour. It takes you through the historic part of town - and is only 45 minutes. They stop at Creek Street to take pictures and to give you a fairly lively explanation of this part of town. There is no walking involved; the trolleys pick you up on the dock. (Hopefully, your ship will be tied up to the dock near Visitor's Bureau. )
The funicular (tram) takes you from the upper end of Creek Street to the Cape Fox Lodge where you can take pictures of the ships at the dock. You must pay a little to go up the tram, but pay nothing to return to Creek Street.
Tickets are sold both on the ship and on the dock.