Congratulations Laura and welcome to the forum! I know it's hard to choose a location with so many great countries and cities to select from. Have you traveled to any beach towns with your fiance before you got engaged? If so, that could be a great place to start. As Sapphire723 said, there are many different locations detailed throughout this forum such as Jamaica, Hawaii, Turks and Caicos, Bahamas, etc. Also, remember, Mexico is a huge country so you could choose a spot that's very different from your friend's wedding.
It can be challenging to select a location but it can also be a lot of fun as well. Happy planning!