Definitely depends on the person. My kids wouldn’t turn down a free all-inclusive vacation resort but definitely wouldn’t be their first or second choice or their style.
Another option might be a cruise. Carnival seems to attract young people and the drink prices might encourage moderation. 
You’re very thoughtful–might be a good idea to have some idea if that’s something he’d welcome before you invest a lot of time and energy. He may have other ideas and plans.
This is great input - thank you all very much!
Think all-inclusive is the way to go. Should give them enough options to have a great time. And if they were in a place that gave them an option to go into town, or generally get away from the resort if they want that would cover all bases. While I would love to do an adventure vacation, not sure that’s what he’d like at that time. His last semester is going to be a slog, and he’s going to want some time to decompress.
@HImom your point is well taken. Will price out a few of these before going further to see what kind of budget this trip will need. Hopefully that doesn’t end the conversation lol. When he’s home this summer will broach it to him. Show a couple options to give him some ideas then let him loose to do his own research on locations, resorts, etc. Or if he’s meh about it, oh well… Plan B for graduation gift. No idea what Plan B is though 
If you want a specific island, I would pick Aruba or St Thomas/St John.
However I would pick a cruise instead. Pick something like Royal Carribean that leaves and returns to Port Canaveral and they visit the Carribean islands. They have five star food which is also included. It’s usually cheaper too. Lots to do that way!
The only cruises I’d recommend are Carnival, Royal, Princess and Celebrity. The last two are more upscale than the first two. Princess is my favorite but they are all good. Just try to pick a cruise line that’s newer. Also Carnival usually has a younger crowd but in December they probably all have similar crowds.
I’ve cruised 13+ times so far and in my opinion it’s the best way to vacation. I’ve been to Aruba St Johns via cruising a lot of times and it’s great.
Any questions just ask.
Just thinking, do all friends have current passports? If not, I would stick to USA, Puerto Rico, USVI. What about North or South Carolina?
Son’s passport is current, several years from expiration. Don’t know which friend he’d choose so that’s something to decide this summer so easily enough time to get passport. Thanks for thinking of that.
Reluctant to do a cruise. Not sure exactly why, but I don’t see him jumping on that.
North and South Carolina won’t be warm enough in January. Actually, can’t think of anyplace with a beach in continental US that’s likely to be warm then. We’ve been to Key West about that time and it was quite chilly at times.
Oh I missed that it would be in January. I thought it was just around the corner. Sorry. Plenty of time to plan ahead!