I would suggest booking excursions in advance, seeing as you are travelling in school holidays, when it will be maximum capacity onboard.
It is not essential to prebook trips but if you have a particular `must do` trip in mind it does no harm and you are then sure of your place. Normally there is no problem as long as you book one to two days in advance on the ship.
Since many tours leave as soon as the ship docks, it would only be half day tours that you would realistically be able to "book on the day" IF there was any space on them! If there are any tours that you are sure you want to take, then book them up ASAP. Spaces are limited on most tours, and once these go then that is it! Many people find that the tours they want to book are full if they wait until they board the ship. Before you book anything DO SOME RESEARCH! Can you visit the same places that the tour does independently? A classic example of this is in Madeira. The Wine Cellars, where you can sample the tipple that the island is famous for, is a feature on many tours here. It is actually situated opposite where the ship docks, a short walk away! Find out how much a Taxi would cost, or how about a hire car would be for the day? I always hire a car in the canaries. They can usually be booked on the ship, and will be waiting for you on the quayside when you dock! Tours are very expensive, and often include bits that you are not interested in. Don't forget, on a tour you pay per person, the hire of a car or taxi is per vehicle! Try getting together with another couple with the same tastes as you to share with. Hope this has been of some help. Have a great cruise. Happy cruising.