From the port at Athinios or the old port below Fira, boat trips run to the volcanic islands in the flooded caldera. Nea Kameni is an active lava islet where you can walk to the steaming summit crater — a 20-minute hike on bare black rock with views back to the caldera cliffs. Palea Kameni, nearby, has a hot spring where you can swim in warm, sulphurous orange water. The experience is strange and memorable in equal measure.
Half-day tours are the standard option, departing from Fira's old port (accessible by cable car, donkey, or the 588-step staircase) or by larger ferries from Athinios. Many tours also stop at the small inhabited island of Thirassia for lunch. Full-day sailboat trips circling the caldera are a more leisurely alternative and worth the extra cost for small groups.
Sunset cruises are popular but pricier. If you are booking independently rather than through a hotel, go to the port and compare prices directly — there is considerable variation.