For those that are concerned that the dreaded volcanic ash may disrupt their journey to Symi, I can assure you that it is possible to get there without catching a plane at all. I do sympathise with this approach having lost 4 days of my spring holiday on Symi to the ash clouds.
If you want to do the whole journey by public transport, which is quite possible as long as you are happy to let the journey itself be part of the holiday experience, the definitive guide is produced on-line by The Man In Seat 61. He shows you how to get to Athens by rail or rail and ferry from many parts of Europe, and the site is updated regularily. I can also recommend it as a resource for long distance rail travel throughout Europe.
Once in Athens, you can catch either ANEK’s Tuesday and Friday overnight departures from Piraeus Port which make their way to Symi lunchtime or early afternoon the following day,en route to Rhodes; or you can catch Blue Star’s much faster overnight service which runs daily and if on time, gets to Rhodes in time to catch the morning departures to Symi (though not on Saturday nights when it leaves later and arrives later). I say if on time because it is risky to assume that these ferries won’t get delayed loading and unloading vehicles at island stops along the way. Hellenic Seaways are also about to enter the Piraeus-Rhodes market with a newly-rebuilt ferry called the Nissos Rodos.
Both ANEK and Blue Star offer a variety of sizes of private or shared cabins, or reclining-seat lounges, or benches on deck. HSW are likely to do the same.
You can of course drive to one of the Italian ports (Venice, Ancona, Brindisi, or Bari) and take your car onboard the ferry to Patras in Greece, drive to Piraeus and catch the ferry to Symi, or Rhodes and then Symi. This is going to be expensive in ferry charges for the car, you’ll spend a lot of time making ferry bookings, and you are then left with the problem of parking it on Symi. If you look at the photos of Symi, you’ll see the problem. Houses climbing up very steep hillsides, with access designed for people and donkeys. This is what gives the place its beauty, but there are very few roads and even fewer parking places for tourists. Driving is really more for those who are seasonal residents of Symi and need to travel with several months worth of belongings, and often, family pets, who will be less stressed by being with “their” people the whole way instead of stuck in an aircraft hold.
To avoid potential problems with customs officers, it is better to stay wholly within the EU or EEA when making the journey (as this way you won’t meet any) – so try to avoid Albania/Bosnia/Croatia/Former Yugoslav Republic of Macedonia/Montenegro/Serbia unless you have plenty of time, no pets, and are travelling light. The scenery is spectacular, the people friendly, but until some of these countries join the EU (and they are applying) routes through Italy or Bulgaria have many advantages. Certainly ignore the results I got from my Satnav, which routed me from the UK to Turkey, over the Bosphorus Bridge, down to Marmaris, and then Rhodes to Symi, taking (allegedly) 42 hours, presumably not allowing for fuel stops, food stops, delays at borders, or sleep.