Exumas are stunning, absolutely!! August is not so reliable for the wind, though. You may get lucky and have some afternoon breezes - if you're set on Exumas, best to watch the forecast then book accordingly. Flights from Miami are pretty inexpensive and quite easy!
Chris is a local there - I hope he chimes in, here...
Yes- Bonaire is a good option. Beautiful and friendly island, and some of the best snorkeling/scuba diving in the world! Water is flat, and you can take a little trip to Klein Bonaire for a day, if you wish. The local kite operation is excellent, and the guys very friendly! http://www.kiteboardingbonaire.com
. Flights from Miami to Bonaire are not as inexpensive as some other destinations.
Puerto Rico- yet another good option!! Summer winds are usually quite good, and you can find lots of great kite beaches! San Juan itself, of course, is an option. You can also head east to Fajardo and grab a fishing boat to take you to Palominito Island for a real treat! (Advisement: Pay when they pick you up, not before!!) Or, La Parguera is a flat water dream! Lots of additional options, too. Flights from Miami are abundant and can be very inexpensive!
In general the Caribbean gets good Trades in August, but some islands are more wallet-friendly than others, so don't let a lesser-expensive flight be the deciding factor.