The Sporades Islands have a quite different atmosphere from the travel-poster Greece of the Cyclades: instead of bare rugged hills topped with dazzling cubistic villages, the Sporades are fertile and heavily wooded, with pine-forested mountains sloping down to some of the best beaches in the Aegean, and the local architecture is more characteristic of Northern Greece, with less whitewashing and more stone roofs. Skiathos is the best known and easiest to get to; accordingly it is the most crowded, with throngs of visitors arriving all summer. Skopelos, while just as attractive, is less touristed outside of July and August, while Alonnisos is much quieter.
From Athens you can buy a combined bus/ferry ticket which will take you overland to the port of Agios Constantinos, where you can transfer to a boat to all three Sporades islands. If you have your own car, you can also drive to Agios Constantinos and take a car ferry from there, but bear in mind that not all scheduled boats carry cars, so you'll need to verify in advance which ones do. There are also some boats to the Sporades from the mainland port of Volos and from Kimi on the island of Evvia.
Only Skiathos has a commercial airport, but once you get there you'll find frequent boats to the other Sporades.