Unlike the Caribbean, for example, there are fewer tour guides touting for business in European ports.
If, for example, you want to see the Alhambra in Grenada, there is no point looking for a tour when you get off the ship in Malaga.
You need to do some research in advance, decide what you want to do, and then look for tours, and pre-book.
However, nothing wrong with just wandering around Malaga, Seville, Marseilles etc, getting a little lost, having a drink in a pavement cafe, and just relaxing.
It is not compulsory to visit as many tourist sights as possible!