IMO, visiting Rhodes is a must do. We went last year for only two nights and will return this year for probably 4. The castle is one of the best I've ever toured.
As for getting into Mandraki, it is best to hire an agent. It cost about 70 euro extra for the agents work
over trying to get a slip on your own and was well worth it to know you have a spot. And they do all the paperwork for you. I used Kronos Agency and they did a nice job.