Questions like this tend to solicit a random splattering of "I like this place, it's the best" kind of responses - everyone will have their perfectly valid reasons for liking one location over another, unfortunately their reasons may not be important to you. You will hear from the Aruba crowd, the St. Maarten crowd, the Cayman crowd and so on.
All of what follows should be considered to be safe provided you use normal precautions and clean subject to the accommodations you select and areas you travel to - every place will have good and not so good. If speaking english is important you'll get that in each place as well. Some places will give you a more sanitized version of their original culture as a result of the influx of foreign tourists - if it's important for you to experience a different culture you'd be well advised to steer clear of what follows since they've all struggled to retain their cultures under the economic weight of the tourist industry.
- Aruba meets all of your stated requirements although a bit light on snorkeling and a days worth of sightseeing will cover the whole island. Nightlife is okay, plenty of casinos in the hotel district. We found the beaches very crowded. We found the average age to be north of 40.
- Cancun meets all of your stated requirements provided you can trade into one of the Royal Resorts, after that the accommodations can get a bit tricky. It has the most superior site seeing of the 4 listed if you are interested in Mayan ruins but also has other interesting options available. Swimming can be a challenge given the strength and size of the waves. Beaches are crowded but not quite as bad as Aruba. Cheapest of the 4 listed. You will find many people your age and younger.
- St. Maarten meets all your requirements however the accommodations might be a notch lower depending on where you are able to trade into. Site seeing is limited but the island has many, many fine beaches some of which will have almost nobody on them. Quite a mixture of age groups and depending on the TS you stay at you can get one extreme or the other.
- Grand Cayman meets all of your requirements however is lacking on nightlife. Site seeing is about a day or so worth similar to Aruba (really depends on what you want to do), has the best snorkeling of all of the locations listed, has amazing diving if you are game to give that a try. Has excellent beaches as well and if you are on the east end they are pretty much empty. Most expensive of the 4 listed. A reasonable mixture of ages but mostly north of 30.
Personally, I think you'd enjoy any of these but only you know how much relative importance you place on the key elements you've listed. My observations reflect only our personal experiences having visited all of these locations and of course may not be representative of what others may have encountered.
Enjoy!
What island and resort would you recommend for a couple--age 25 and 27--for their first visit to the Caribbean? They want to spend time on the beach, snorkel, go sightseeing if there's anything to see, have nice restaurants and night life, and be in a safe, clean area.
They would travel around the end of February, beginning of March.
