Incredibly good value transport card from the Tidpunkten: 100 SEK for trains, trams, buses and ferries throughout Goteborg and Bohuslan. Buys a number of kupong (currently 18). Insert the card in the machine on board and press the required number of kupong (minimum 2). Permits onward travel in the next hour, longer for areas outside Goteborg City. Long journeys (3 hours or more) are 8-10 kupong (£4-5), even on the express coaches.
Combined prices for multi-mode journeys are a lot cheaper than paying for individual elements; generally the extra cost of local onward travel from a ferry by bus is one kupong.
Use it to wander round the coastal islands. If you are there for the weekend, one ticket can be used for several people including travel on the much cheaper bus from/to the airport if you are using Ryanair; the bus stop is at the end of the Airport road about 300 metres from the departure lounge.
www.vasttrafik.se/eng/internet.aspx?desktop=177&menu=1&language=1053