Bonaire, Aruba or Curacao average a high of about 89 degrees in July; cooler than Charlotte. Bonaire is difficult to get to and not very pretty, nor with big hotel options. But the diving is fantastic.

I'm not as familiar with Aruba or Curacao, but they are more touristy destinations.

Actually, the Caribbean is not awful compared to the US South. In fact, it is often much more comfortable because of the sea breezes. (I'd compare it to going to Myrtle Beach in July.)

CO has flights to Bonaire from EWR and IAH, neither as good as leaving from CLT. Though, you could transfer AMEX to CO and then I think use your UA miles.

OTOH, Bermuda is a great suggestion. I frankly would stay away from European beach destinations. But, that's just me.