First time to Greece. Plan is to visit Athens first the 2nd week of April. From reading the closest island for Delos is Mykonos. It looks as though there are not many convenient ferry times so would be be best to fly to Mykonos? Would spend several days in Mykonos. Sounds like many places are closed in April. Not planning on going to the beach or lots of night life. Goal is Delos. Is an organized tour better for Delos? Any suggestions? Thanks.
In the 2nd week of April there will be 2 ferries per day to Mykonos from the port of Piraeus and at least 3 ferries from the port of Rafina.
There are also daily flights with Aegean/Olympic or Sky Express
Plane or ferry, it's up to you.
To visit Delos see with Delos Tour: Visits begin in April. It can be done with a guide which I recommend since you are very interested in Delos.
If you want to avoid the Mykonos exorbitant prices and night club atmosphere, the visit to Delos could be done from Naxos or Paros, (they include a stop of a few hours in Mykonos) but I do not know on what exact date they start, usually it's not until May