Remember that hotels and other prices will be at their highest at peak holiday time.
I like SXM in the Caribbean as far as UA destinations go. You can easily get to St. Bart's and Anguilla from there, too.
Antigua is beautiful, and a visit to Montserrat from there is interesting. St. Lucia is a spectacular island.
My favorite is Barbados, but UA doesn't fly there.
Have fun!