Skiathos is famous for its beaches, and I'll cover the most important ones briefly, proceeding clockwise from the port. The most popular beaches are west of town along 12km (8 miles) of coastal highway. At most of them, you can rent an umbrella and two chairs for about 15€ per day.
The first, Megali Ammos, is the sandy strip below the popular package-tour community of Ftelia. It's so close to town and so packed with groups it probably won't appeal to most. Vassilias and Achladias are also crowded and developed; Tzanerias and Nostos are slight improvements. Farther out on the Kalamaki peninsula, south of the highway, Kanapitsa is good for fans of watersports; Kanapitsa Water-Sport Center (tel. 24270/21-298) has water and jet skis, windsurfing, air chairs, sailing, and speedboat hire. Scuba divers will want to stop at Dolphin Diving Center (tel. 24270/21-599) at the big Nostos Hotel.
Across the peninsula, Vromolimnos (Dirty Lake) is fairly attractive and relatively uncrowded, perhaps because of its unsavory name and the cloudy (but not polluted) water. The beach offers water-skiing and windsurfing. Koulos and Ayia Paraskevi are fairly well regarded. Platanias, the next major beach, isn't crowded, perhaps because the big resort hotels here have their own pools and sun decks. Past the next headland, Troulos is one of the prettiest beaches, due to its relative isolation, crescent shape, and the islets that guard the small bay. Nearby is Victoria Leisure Center (tel. 24270/49-467), which has rooms to rent, a pool, shops, and two tennis courts.
The last bus stop is at much ballyhooed Koukounaries, 16km (10 miles) from Skiathos town. The bus chugs uphill past the Pallas Hotel luxury resort, then descends and winds alongside the inland waterway, Lake Strofilias, stopping at the edge of a fragrant pine forest. Koukounaries means "pine cones" in Greek, and behind this grove of trees is a half-mile-long stretch of fine gold sand in a half-moon-shaped cove. Tucked into the evergreen fold are some changing rooms, a small snack bar, and the concessionaires for beach chairs, umbrellas, and windsurfers. The beach can be extremely crowded but with an easy mix of families and singles. (There are several hotels near the beach, but because of the intense mosquito activity and construction, I recommend staying back in town or along the coast road.)
Ayia Eleni, a short but scenic walk from the Koukounaries bus stop (the end of the line) west across the island's tip, is a broad cove popular for windsurfing, as the wind is a bit stronger than at the south-coast beaches but not nearly as gusty as at the north. Across the peninsula, at the far right end of the beach, 15 to 20 minutes of fairly steep grade from the Koukounaries bus stop, is Banana Beach (sometimes called Krassa). It's slightly less crowded than Koukounaries, but with the same sand and pine trees. There's a snack bar or two, plus chairs, umbrellas, windsurfers, and jet skis for rent. One stretch of Banana Beach is the island's most fashionable nude beach.
Limonki Xerxes, also called Mandraki, north across the island's tip, a 20-minute walk up the path opposite the Lake Strofilias bus stop, is the cove where Xerxes brought in 10 triremes (galleys) to conquer the Hellenic fleet moored at Skiathos during the Persian Wars. It's a pristine and relatively secluded beach. Elia, east across the little peninsula, is also quite nice. Both beaches have small refreshment kiosks.
Continuing along the northeast coast from Mandraki, you arrive at Megalos Aselinos, a windy beach where free camping has taken root. It is linked to the southern coastal highway via the road that leads to the Panagia Kounistria monastery . You must continue north when the main road forks off to the right toward the monastery. There's an official campsite and a good taverna. Mikros Aselinos, farther east, is smaller and quieter, and you can reach it via a dirt road that leads off to the left just before the monastery.
Skiathos's north coast is much more rugged and scenic, with steep cliffs, pine forests, rocky hills, and caves. Most of these beaches are accessible only by boat, and of these, one is well worth the effort: Lalaria, on the island's northern tip, is one of Greece's most picturesque beaches (although it is a pebble beach). One of its unique qualities is the Tripia Petra, perforated rock cliffs that jut into the sea on both sides of the cove. These have been worn through by the wind and the waves to form archways. You can lie on the gleaming white pebbles and admire the neon-blue Aegean and cloudless sky through their openings. The water at Lalaria is an especially vivid shade of aquamarine because of the reflective white pebbles and marble and limestone slabs that coat the sea bottom. The swimming is excellent, but the undertow can be quite strong; inexperienced swimmers should not venture far. There are several naturally carved caves in the cliff wall that lines the beach, providing privacy or shade for those who have had too much sun. Lalaria is reached by caique excursions from the port; the fare is about 40€ for an around-the-island trip, which usually includes a stop for lunch (not included in the fare) at one of the other beaches along the northwest coast.
Three of the island's most spectacular grottoes -- Skotini, Glazia, and Halkini -- are just east of Lalaria. Spilia Skotini is particularly impressive, a fantastic 6m-high (20-ft.) sea cave reached through a narrow crevice in the cliff wall just wide enough for caiques to squeeze through. Seagulls drift above you in the cave's cool darkness, while below, fish swim in the 9m (30-ft.) subsurface area. Erosion has created spectacular scenery and many sandy coves along the north and east coasts, though none are as beautiful or well sheltered from the meltemi (high winds) as Lalaria beach.