A safe environment with a low crime rate!
And even with resorts and "romantic" settings, the Caribbean has a lot to offer solo travelers too! Enjoy a pampering resort on your own, and join an organized tour and make new friends!
Factors that are considered are safety and crime level, solo female travel safety, solo travel friendly activities, possibility for small group tours to socialize!
Including for women, although, solo female travelers do get a significant level of attention in Cuba. Not dangerous, and whether it is nice or annoying, depends on you!