My top recommendation would be San Juan, Puerto Rico. It offers a wonderful mix of history and culture and nightlife and great food, alongside excellent beaches and abundant activities. Hotels and rental apartments are mostly in the mid-range for the Caribbean, but food and drinks and most other things are great value. You could get a place in the Condado area, which is close to the lovely Old San Juan neighborhood and plenty of nightlife all around. And then perhaps you could rent a car and stay for a few days at one of the smaller beach towns elsewhere on the island. You wouldn’t even need a passport, although it does feel fairly exotic while you are there. But because it’s part of the US, it’s also well organized and safe. Most people you’ll meet will speak at least some English, and many are fluent, so it’s easy even if you don’t speak Spanish.
In addition to the obvious perks of pleasant weather and fewer tourists, off-season rates at accommodations are abundant. Voted No. 1 continental U.S. island by Travel + Leisure readers for the third time in 2018, Hilton Head has much to offer visitors. One of the most popular activities, perfect with autumn temperatures, is biking on the more than 60 miles of pathways covering the island, including along 12 miles of beaches. Bird watchers will keep busy spotting egrets, sandpipers, wood storks, sea gulls, pelicans, and ospreys. Well known as a golfer’s paradise, the Hilton Head Island area boasts more than 33 courses.
This year I’ve based myself in Miami, USA, primarily so I can visit many places in the Caribbean that I hadn’t been to before. I just got back from a week in Jamaica and soon I’m going to Puerto Rico, Dominican Republic, Panama, and Colombia, as mentioned. But in December I’m hitting the road again and I’ll be in Asia for the winter and probably in London for much of 2016. Sorry I don’t have more specific information to help you plan your trip, but it sounds like you’ve got some really good choices already under consideration. And I’m sure you’ll have a great time. -Roger
Interestingly, I had not seen that June travel warning. It’s hard to know what to think of it. On one hand, the US State Department has official warnings like this to many places, including quite a few that people safely travel to every day. And also, even when Colombia had its problems for all those years, Cartagena was considered safe because it’s this tourist beach town and the drug and FARC problems had nothing to do with it. My best guess is that Cartagena itself would be fine and perfectly safe, but I wouldn’t rent a car and drive around the highways myself at this point. Sadly, I think it might be best to save Cartagena for another time when there is no known threat.
While Cancun has become the king of the Mexican beach resort towns in the few decades since it was formed, it's only one of many, and all of them on both coasts have very nice weather in December. In fact, Cancun is probably the most expensive of them, so especially if you are coming from the west coast you are better off in Puerto Vallarta, Acapulco, or Los Cabos instead.
But if you wanted more to explore the area near Singapore then Malaysia and Thailand are the obvious choices. The three most popular stops in Malaysia are Malacca, Kuala Lumpur, and George Town on the island of Penang. I quite like all of those so it’s hard to recommend one over another. I really like Kuala Lumpur and have spent quite a bit of time there, but honestly compared to Singapore it seems a bit untidy and old fashioned. In other words, if you are tired of a big and busy city like Singapore, then don’t plan much time in KL. Malacca and George Town are both smaller tourist cities with great food and interesting sights. There is frequent and cheap bus service from Singapore going through Malacca and onto KL. Then more buses from there to Penang and onto Bangkok.
You would probably get some rain in Cartagena in November, but mostly in the first half of the month, and even then it tends to come down in short bursts rather than all day, so it’s usually easy to avoid. I love Cartagena for trips like this because the walled historic part of the city is really lovely and fun, with plenty to see and do. The nearby beaches are big, but the sand isn’t white and fluffy, so they won’t make too many “best beaches” lists. It’s also very cheap there, and especially at the tail end of the off season like that. As long as you don’t mind the possibility of a couple of quick rain storms, it should be great.
You still get warm ocean breezes and 70-degree temperatures, plus the Hawaiian islands go all out for the holidays. Locals host potlatch-style parties where the Christmas goose is replaced by a luau pig, and the resorts deck themselves out like tropical North Poles, including a massive tree-lighting ceremony at the Outrigger on Oahu and tree-decorating contests at Mauna Kea on the Big Island. Residents there climb to the top of the 13,000-foot-tall volcano, shovel snow into their trucks and speed back down to build snowmen in their yards before it melts.