Last minute honeymoon booking – need your suggestions

My older sister is getting married in 3 weeks and the cost has come out to much more than her and her fiance were anticipating. It's to the point that they have cut out having a honeymoon. It saddens me because my sister works hard in a high stress job and hasn't had a break in many years so I would like to surprise them with a honeymoon. My budget is $1500-2000 per person to an all-inclusive in Mexico, the Caribbean, or South America flying out from Toronto. They are in their early to mid-30s, aren't much of the party types, and would probably appreciate an adults only resort. I have tried to do the research on my own but haven't experienced much Caribbean travel. I thought it might be better to get some suggestions based on first-hand experience.