You will want to think about what “extras” you want to do onboard. Most ships will have a spa and specialty restaurants, at which reservations are strongly recommended. There will also be shore excursions that are available on a first come/first served basis. You may consider booking these in advance.
Each cruise line has its own procedure for how these are done. In most cases, your reservation must be paid in full before you can prebook these things. With Disney for example, you must not only be paid in advance, but you must also be with a certain date prior to your cruise. This date depends on the number of times you have cruised with Disney in the past (see Repeat Cruiser section below).
With some cruise lines, you will have to prepay for what you reserve in advance. This was true with Celebrity when we cruised earlier this year. With other cruise lines, you simply make a reservation and then once onboard, you will be charged. You will want to check what the cancelation policy is with the shore excursions, spa appointments and specialty restaurant reservations that you make.
Some cruise lines will not allow you to prebook certain things online. With Disney, you cannot book some of the spa appointments and the cabana rental on Castaway Cay online in advance, it must be done on the ship. On some cruiselines, you can also book shows online in advance. For example, on Royal Caribbean’s Oasis of the Seas, you can prebook some of the shows in advance, eliminating the need to stand by for available seats at the start of the shows.
Unless you have plans in each port of call or have your heart set on a particular spa offering, you may consider waiting until you get onboard to see what Port Day specials are being offered by the spa. You can save quite a bit off the cost of the spa activities if you can go while the ship is in port.
Some of the cruise lines have areas, usually for adults only, with steam rooms, saunas, whirlpools, and relaxation areas. These areas will have a fee for daily use or even a cruise-long pass. Some of these areas will have limitations on the number of people that can sign up, so you will want to do this as soon as possible. On some ships that have Spa Suites, this amenity can be included.
You may also consider shore excursions offered by outside vendors. You should be cautioned on this. Often the pricing will be less than similar (or even identical) excursions offered by the cruise line. You will want to make sure that you are covered if something happens and your ship does not arrive at the scheduled port of call. Some vendors will still charge you so you will want to make sure this is not the case.
Additionally, when you are on a shore excursion, delays can happen. On our last cruise, we were due to get back into port 30 minutes prior to all aboard time. However, this did not happen. We arrived back at the pier about 5 minutes prior to the all aboard time. Had we arrived late and booked with the cruise line, they would have held the ship at the pier for our arrival. We witnessed this happening in Palermo Sicily on the Disney Magic. However, if we had booked the excursion on our own with an outside vendor, the ship would not wait for us to get back. It would be our responsibility (and expense) to get from the port of call back to the ship, likely at the next port of call.
I generally won’t do an excursion independently unless it is in a port I have been to before and the plan is to be back to the ship at least one hour or more before all aboard time.