Thailand has dozens of beautiful islands and beaches to the south of Bangkok.  Some may be reached direct by plane, others by bus or even overnight train.   There are islands to everyone's taste so please ask for further details about your requirements.  Ko Samet is the nearest island to Bangkok and can be reached by bus and small ferry whilst Phi Phi for example requires a little extra effort.  Both Phuket and Samui are well served by flights from Bangkok.