Although it is one of the smaller islands of the archipelago, Avşa is the most heavily touristed one. In fact, it was one of the most popular holiday destinations in 1970s for mid-class families in Turkey.
The island is officially named Türkeli, but this never caught on, and most everyone, except perhaps the government documents, keep calling it Avşa, which derives from the Greek name of the island, Afissia.
Roughly in the shape of an hourglass, the island has two settlements: the main town, also known as Avşa, centred on the western cove and the smaller village of Araplar, officially Yiğitler, occupying the eastern cove.
The island has a large range of hills running down the middle, some beautiful bays for bathing, swimming or just relaxing in and gets very hot in the summer.
English is not a popular language, but many people will speak some English or German. This is mainly an island visited by Turks rather than foreigners, but Turks are incredibly hospitable people and don't be surprised if someone is sent out to fetch an English speaker to translate if there isn't one there in the shop.
Year-round boats which can accommodate cars from Tekirdağ (on the northern/European coast of the Sea of Marmara) and Erdek (on the southern/Asian coast of the Sea of Marmara, 1hr 45min). There are also pessenger-only fast ferries  (3½hr) from Istanbul during summer months. Ferries from Tekirdağ cost 15 TL/person, while ferries from Istanbul cost 40 TL/person (or 30 TL/person on the slower ferry departing on Saturday mornings).
Buses leave from Avşa's main square in town and travel around the island, there are also a couple of dolmuş operators, kind of a cross between a taxi and a minibus. These follow particular routes around the island.
It is possible to walk around the island but it's a good stretch of the legs and not for the unfit. If you do walk, make sure you take water with you — some parts of the island are very rural and you won't see another person for miles or hours.
Cycling is also an option, but please bear in mind that Turks tend to see road safety laws as an optional hindrance rather than a hard rule.
You can take a scooter or car on the ferry if you want, the roads are variable. Unfortunately the government in power is not providing enough money to the municipality to maintain all of the roads, although there has been a programme of improvement over the past few years.
As you might expect for an idyllic Mediterranean island paradise, there's not a huge amount to do on the island. In the summer this is just as well as it gets really hot.
In Avşa there's a "Lunapark" fair. There's also sandy beaches all around the island, some public, some private. There are a few real gems with some natural bays due to the island's shape. It's possible to walk over the main hills fairly easily, but make sure you take plenty of water if you do and either set off very early in the morning, or in the mid-afternoon.
The main strip in town might look at little tacky, but if you go up to the ferry port, nearby there's a pair of fish market stalls. The guys there are more than happy to show you how they handle the fish, the fish are fresh and great cooked in a Turkish barbeque (mangal).
Heading down the strip and out you'll come to a place called Mavikoy. Before heading up the hill there's a restaurant with a sea view, some nice beaches and past the hill is a lookout point sometimes frequented by couples around sunrise/sunset.
Island’s local wine is well known and of variable quality. Bortaçina (Liman Mevkii, Yiğitler, ☎ +90 266 892-10-03 (firstname.lastname@example.org, fax: +90 266 892-10-02), . ) is a local producer well-known also in the mainland, offering wine-tasting (and a grilled meat and fish menu) in its renovated winery in Yiğitler.
If you don't like wine, there's a reasonable selection of beers in the various restaurants and bars in Avsa, but you might be better off visiting the local supermarkets. It goes without saying that Turkish spirits such as Raki are freely available, but you might struggle to find some foreign spirits, such as Gin or Rum. Expect Whiskey to be limited to Jack Daniels and Southern Comfort. Bear in mind that Avsa gets extremely hot, and you might want to stick to the Efes Beer.
There are lots of guesthouses (pansiyon) on the island, so finding somewhere to sleep shouldn’t be a problem.
Island’s telephone code is 266 (+90 266 when calling from out of Turkey).
While the island is generally a fairly crime free place with lovely people, you should take care to keep an eye on your belongings at the beach, sometimes people do have things stolen (although this isn't a common occurence). It's best to avoid doing anything stupid, like getting drunk in the night clubs. You should also be aware that anything imported to the island is generally a bit more expensive, particularly in restaurants. Don't be surprised if you're charged a small fortune for drinks in bars or clubs, or for food in restaurants at peak season.
The island gets very very hot in the summer and can get very windy too. Make sure you carry water with you during the summer. The island has a seawater desalination plant, so the water is generally safe to drink from the tap but most places will serve bottled water fairly cheaply, as until recently it had to be imported anyway. If you're walking around the hills try not to go when it's windy or when it's very hot, it's easy to go off the beaten track and end up somewhere miles away from anyone. As a general rule as long as you can get to a road, head to the shore and then walk to the nearest civilisation.
The island is a fairly laid back progressive place, but be careful with public displays of affection, clothing and drinking. Drinking is acceptable, getting raucously drunk and being loud and/or offensive less so. Girls may find themselves the source of unwanted attention on the beach from boys if not accompanied by a man, especially girls that look foreign (as this is uncommon). As with all of Turkey, during Ramadan things get a little stricter and a little frustrating. While most places will sell alcohol during Ramadan, it's best not to drink in public if you can avoid it. Likewise, during Ramadan women should dress a bit more modestly than normal.
If you're not Turkish, you may find some cultural customs a bit strange. If something happens and you don't agree with it (e.g. an old lady picks up your child and starts cooing at them, or a retailer offers them sweets) it's not intentional. Turkish people are very passionate and while Avşa is a fairly heavily populated place in the summer, the people have a more rural mentality compared to Istanbul. As such, you may find things like the hospitality a little overwhelming at first, but the people are just proud of their island and want to make sure your stay is a happy one.
Other islands of Marmara Archipelago.