Johnny Rockets milkshakes are covered under the deluxe and refreshment drink plans.
To answer the other question,yes, you can wait until board to book shows and dining once on board through concierge IF you are really flexible. If not, you may want to do it before you board.
If you don't want to do it through cruise planner, you will get an email from your concierge several days before the cruise. You can ask them to make those reservations then.