If you have less than a week, you are probably restricted to the Bahamas, Mexico, parts of the Caribbean, or a "cruise to nowhere". A "cruise to nowhere" allows passengers to cruise for a long weekend out of port into the ocean and then return. No ports of call are made by the ship, but you can get a feel for what a cruise is like.
A week's vacation will open up the Caribbean to you, and depending on where you live you can even get to Europe, Hawaii, Alaska, or the South Pacific.
If you can squeeze out ten days to two weeks, you will have many more options--including all seven of the continents.