Please don't say Vagas -- been there, done that. Getting laid is not top priority, just want to get away and have some fun.
What is your priority then? And what time of year are you going? Europe's a great place for a single traveler: excellent public transportation; plenty of bars/restaurants; easy to meet people because of your "accent." Barcelona, Paris, Nice, Berlin, Prague, Rome...they're all great, and don't necessarily limit yourself to my list.
My vote is for Cabo San Lucas. It has the partying if you're in the mood, but also has lots to do for the laid back type. The golf is pretty good as well.
Try a place you might not ordinarily go - Thailand, Singapore, China, Egypt - someplace that has a lot of history will give you plenty of things to do...
Easy, do something that you wouldn't be able to do with a group of friends or family. If you're into that sort of stuff, try Tanzania and climb Mt. Kilimanjaro. You'll likely go in a group and meet some amazing people on the way. Afterwards, you'll have a much more interesting story to tell.
If you're into quiet, rest and relaxation on your vacations, rent a beach home (http://www.homeaway.com), explore and get surfing lessons.
Trips across Europe and alcohol fuelled adventures are always more fun with friends.
For me, the answer is get outdoors and rough it. Find a national park near you and go there. You can camp, canoe, fish, bike, hike, grill, build a fire...basically whatever you want. Best of all...it's pretty cheap.
Just mind the bears.
I agree with the outdoors stuff-try the mountains, Snow Skiing can be a great solo-vacation as you'll surely meet up with people on the lifts and the bars afterwards.
If you're into sports, it's cool to go to your fave baseball team play a weekend series. You can go and watch the games, have a beer, and just hang around whatever town they're in. I just tested it out for last weekend, and it was a great getaway.
One more vote for an outdoorsy vacation. I would make a getaway by myself based on something I'm really passionate about so I don't get too bored sitting around (golf, tennis, surfing, etc.). Or maybe even something you would like to get good at? I'm an avid tennis player, and I love tropical places, so I would research a tennis academy somewhere in the Caribbean or something and go from there.
I agree with what the others have said, Europe might be more fun with friends.
Of course if you just want to chill and not do any activities, there are plenty of places that are beautiful without much emphasis on nightlife/partying/getting laid. Try Belize? Or Grand Cayman? (I'm partial to the tropics!)
If you are 18-35 go on a Contiki tour in Europe. You will likely have over 70% women on the tour, most from Australia and it's just an insane party for 2-3 weeks.
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Beirut- very friendly people, the best party scene on earth and the hottest women on the planet.
Southeast Asia is amazing. I've done it twice now, both times for about 5 weeks. I have a video on my mac account for a quick snapshot of things to do: http://gallery.me.com/rob.snodgrass#100000
Amsterdam is always fun, and has a lot of latitude as far as variety of activities. The tourist turnover is high so the possibility of meeting people is very good. It is basically the ultimate playground. Casinos, red light district, coffee shops, history, shopping. I have been by myself and found it to be a very accommodating city for a single guy. Check out bars at hostels and you will meet like minded people, even if you are not staying at the hostel. Great deals on flights over there as well these days. I would recommend the Heineken brewery tour, renting a bike, checking out the coffee shops, Red Light is interesting. It is also safe. Some of the spots mentioned on here can get seedy if you are by yourself - this gives you some potentially awesome adventures without ending up on "Locked up Abroad."
Just a thought.
This depends a lot on your interests.
I myself am planning some travel the beginning of 2010. I have thought about the following places:
Southeast Asia:
Thailand and surrounding area... think the movie "The Beach" with Leonardo DiCaprio. Strong value for the money, can stay long term (maximizing value of high price airfare) and beautiful surroundings, tropical and very exotic. Large amount of European travelers as well, so able to meet like minded people.
Europe:
Obvious attractions and culture, although more funds required depending on what you do. Lots of people, easy to get to other countries when on the continent and relatively safe to travel to.
South America:
Mainly Costa Rica or Panama in mind.... good value for the money due to exchange rates, exotic culture and environment. Some risks, but these countries seem to be very safe relative to the area and closest to the United States.
And the Olympics are coming up, also considering going to Vancouver in British Columbia, Canada to be there while it is going on.
I would recommend somewhere in Brazil. The girls are always good looking and they are one of the few places on earth that still really likes westerners and Americans.
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What is your priority then? And what time of year are you going? Europe's a great place for a single traveler: excellent public transportation; plenty of bars/restaurants; easy to meet people because of your "accent." Barcelona, Paris, Nice, Berlin, Prague, Rome...they're all great, and don't necessarily limit yourself to my list.