Well, that depends on what you want to do and what kind of person you are. If you like to sit around on beaches and check out scenery, I would recommend going to California, southern France or Italy. I've heard Spain is quite nice, as well. Of course, Caribbean Islands are also popular for that sort of thing.

If you like tourism and sight-seeing, well, I think the recommendations become pretty obvious, just go to large, touristic cities. Paris, London, Mardid, Venice, New York, etc, etc.

If you like sports, such as skiing or biking, California is quite nice, as is British Columbia. Of course, the Swiss Alps are probably the epitamy of skiing resorts, but, as everything in Switzerland, it'll be an expensive vacation.

Personally, I don't like doing doing touristic things, but I also am not a fan of lying on a beach all day. I like to see things, walk around, and just look at cities. Some of my fondest memories of vacations were in Paris, which is a great walking city, in my opinion, along with Boston, Vancouver and major cities in Scandanavia (Stockholm, Helsinki, Oslo). North America's West Coast is also a great place for a road Trip, in either Canada (BC) or the USA (Washington, California) if you have a week or so to spend and enjoy road trips.

My two cents,
illu45