Pretty much anywhere you want to go in the world where there is water (ocean, large river), you can cruise. Some obviously will be very small ships or boats, but you can cruise! There are cruise lines that have cruises going to every continent on earth!
The most popular cruise destinations are the Caribbean, Europe and Alaska. There are many different cruise lines and ships with sailings in these areas.
The Caribbean is a year-round destination, though fewer cruise lines sail here in the summer. It can be very hot in the summer months here. With fewer ships here in the summer though, the ports of call won’t be nearly as busy.
Alaska and the Mediterranean is just the opposite. Spring through fall are the popular times for these destinations with few if any sailings available other times during the year. The Mediterranean is a cruise destination that can be very busy and tiring for the cruiser. There are usually many ports of call, sometimes day after day. Additionally, some of the ports of call are far away from the destinations you would like to see (i.e. Civitavecchia is an hour by train away from Rome). You will be spending a lot of time off the ship in the Med.
Alaska however will offer incredible sights both on the ship and on shore. There are many options with Alaska including one way cruises, roundtrip cruises, inside passage, Glacier Park, cruisetours to Denali.
For shorter cruises, there are cruises to the Bahamas from the East Coast. These usually include a stop at Nassau and/or Freeport. Some cruise lines also include a stop at a private island. For those that want a beach day without worrying about paying fees for chairs, umbrellas, etc. this can be a great day!
Other destinations include Northern Europe, Bermuda, Asia, Australia, South America, the Panama Canal, Mexican Riviera and Hawaii. River boats will tour the major rivers through Europe, Russia, Asia, Africa and even the United States with incredible views right from your stateroom! Imagine waking up with a castle right outside your balcony!
Another destination that is popular is a Canada/New England cruise. These normally sail from Baltimore, New York and Boston and go up as far as Quebec. Some are roundtrip, some one way. These are nice to do in the fall so you can experience the fall foliage.