It all depends on what excursions you are going to attend.
If this is a boat trip, you must always bring swimsuits, a towel, sun protection products, hats, mask tubes (if they are not included in the price), photo and video equipment.
On historical excursions, you need to shoe comfortable shoes, wear light and comfortable clothes, take hats, glasses, water, photo and video equipment.
If excursions involve night crossings, do not forget: warm clothes, a pillow for comfort, rations and water.
Some excursions require a passport (especially those that suggest crossing the border).