The flights between Gotland and the mainland are served by several companies. You can travel with Skyways from Bromma as well as from Arlanda, Gotlandsflyg from Bromma and Direktflyg from Linköping. During the summer the Norwegian company Wideroe flies from Sandefjord and Gardemoen in Oslo. New since 2006 is that also City Airline flies between Gothenburg and Visby as well as between Ängelholm and Visby during the summer months. Unfortunately there are no flights between Linköping and Visby during the most intense summer months.
Destination Gotland handles the ferry traffic between Nynäshamn-Visby and between Oskarshamn-Visby. Nowadays they use two new big high-speed ferries, M/S Visby and M/S Gotland (called SF1500 where the number gives the capacity for passengers) and a smaller ferry called HSC Gotlandia (SF 700).
Of course you can also sail here yourself if you have the means. Visby has an excellent guest harbour right in the city centre which means you are close to numerous restaurants and other service. The only drawback for a sailor is the lack of archipelago out here compared to mainland Sweden.
Visby is sometimes included in the big cruise liners' itinerary for Baltic Sea cruises so you might find yourself here with one of those giants. Some anchor outside the harbour itself and ship passengers ashore with their little service boats. I hope you get better weather than in this picture (as you can see from my page we also had a lot of sun). I guess this was just the gods crying since the picture was taken as we were leaving this beautiful island.
You can also sometimes cruise here from Stockholm with either Vikiing Line or Birka cruises (see Stockholm page) when they do special summer cruises to Riga via Gotland.
Visby Old Town is hilly here and there but you can still cycle along the seafront without hills and the bike is a great way to see the villages around the city or just take you back to your hostel or camping at night. Having said all this, I am a bit disappointed with the cycle paths as the whole of Gotland is sold heavily as a cycling destination whereas I often found myself sharing the path with loads of pedestrians or with cars. Still, it is one of the best places I know to bike in and since everyone does it, car drivers are used to them.
You can rent bikes all over the city so there is no shortage or need to book in advance. In fact, as soon as you get off the ferry, there are two rental places in the form of O'Hoj (a pun since "hoj" is slang for bike) and a place next to the tourist office. Further up at Österport, the eastern city gate, there is a rental place right by the gate (Visby Hyrcykel) which became our own favourite as it allowed you to rent on a 24hr basis so we could bike in peace a whole day and return the bikes the following morning. Most places have similar deals and prices so don't worry too much about shopping around - we just had this on the way to/from the Old town. There is yet another place at the Team Sportia sports shop along Östervägen. Not only mens' and ladies' bikes are offered but also children's varieties, trailers and tandems and you can rent from a day to a week with. Some offer mountainbikes too but most have versatile city bikes with 3-5 gears which will do fine for Visby.
The way most people get here is by ferry since then they can bring their car, caravan and so on along on holiday, or all sorts of things should they be locals shopping in mainland Sweden. Gotlanders have a discount on the ferry since they don't have much of a choice if they want to bring their vehicles but the rest of us pay a bit more. The cheapest tickets do not include a seat reservation so there is a bit of a scrum to reach seats in the restaurant area for the 3 hour crossing but this is usually not that much of a problem - should you want to eat, people are used to sharing tables and many also bring their food to their reserved seats. The reserved seats are what I recommend and a popular thing is to book seats in the front lounge where you can see Gotland suddenly appearing at the horizon. The seats are really comfy, a bit like on planes, and if you bring earphones you can listen to the film sometimes shown on board. Otherwise, the information desk will rent you DVD players and Nintendo games to fill up your time. You can also spend it on the sun deck with a beer if the weather is pleasant enough. The ferries go to Visby itself (with its central harbour) and leave from either Nynäshamn (an hour south of Stockholm by commuter train) or Oskarshamn further south. Summertime there are also ferries from Grankullavik on the neighbouring island of Öland these days again. The ferry company site below also offer accommodation packages.
Daily ferry traffic to Gotland leaves from Nynäshamn and Oskarshamn. The high-speed ferry takes you from Nynöshamn to Visby in less than 3 hours. On the big ferries the crossing takes 4 hours from Oskarshamn and 5 hours from Nynäshamn.
From Stockholm Arlanda, Stockholm Bromma and Norrköping there are about 20 departures each day. The flight to Visby takes 40 minutes