Tied with the nearer Cycladic island of Kea, the Sporades are the second closest island group to Athens.
Time wise, its a toss up between the Sporades and say, Mykonos in the Cyclades, which is only 6 hours from Piraeus by regular slow ferry. If you count the time required to get to your embarkation port and the time on the ferry itself they are about equal depending which Sporades island you decide to go to.
The advantages of the Sporades are many; lower prices, lush foliage, beautiful Scandinavians and the Alonissos marine preserve being among them.
Skiros is the closest Sporades island and to reach it you have to get to the port of Kimi in Euboea which is a 3 hr drive and then one hour or so on the ferry. So that's 4 hours. Skiros is inexpensive, has nice beaches, clear waters, good fishing, fewer tourists and excellent seafood. Its much quieter and less annoying than Skiathos.
To reach the other 3 islands of the Sporades, the closest embarkation port from Athens is Aghios Konstandinos or if you are coming from Northern Greece: Volos.
Aghios Konstandinos (see map above top) looks farther away than Kimi but because its reachable via the National Road is actually quicker to get to.
The ferry boat durations are a bit longer however. Skiathos unfortunately, is overdeveloped and overpriced but has some of the best sandy beaches in Greece. Visit off-season to enjoy it best. Skopelos has one of the more attractive island ports and great trails for walking and hiking. Its a green island with excellent honey and sandy beaches.
There are a decent number of nighttime activities and its a favorite haunt of Scandinavian female tourists.
Allonisos boasts the marine preserve among its 9 satellite islets. It also offers many walking and hiking opportunities and sandy beaches.
Its the eco-tourism destination of the Greek islands. Protected species include the mediterranean monk seal and the rare Eleanora falcon. This island has caught on recently particularly with those with yachts.