To get to the actual volcano's crater (right), entails, after a boat ride, a 30 minute hike, uphill, past hot sulfurous ground. Unless you are a geologist and have sturdy boots ... phew, yawn and ouch. Don't step off the path! You'll see signs advertising the tour all over town. And you will also be invited to spend €60 to take a submarine tour of the caldera. The submarine only descends to a depth of 100 feet, so I am not sure you'll see much at all except fish as Santorini's waters, while clean, aren't particularly revealing.
If you decide to skip the aquatic submarine tour then you can save your money for enjoying the magnificent view from your caldera-view traditional cave house or hotel room, with your loved one by your side.
Some of these modern renovated dwellings can accommodate three couples or children and have full kitchens and every amenity including swimming pools. They are spectacular! Ladies call them extremely romantic! If you prefer far fewer crowds consider a caldera view from Oia or Imerovigli north of Fira.
If you are going to Santorini I highly recommend purchasing one of these recently introduced, in English, SKYMAPS from a kiosk in Athens, the airport or wherever you can find one. (www.skymap.gr) They cost about €5 . Skymaps are made for other islands, too, and have more info and maps on the back. I am not affiliated with them in any way, shape or form.