Sweden is large — roughly the size of California — so how you travel depends on how much ground you want to cover.
By Train
The national rail operator SJ runs trains between most major cities. The Stockholm–Gothenburg route (around 3 hours on the fast X2000) is one of the most travelled, and booking in advance can bring prices down considerably. For regional travel, tickets are sold through regional operators but can often be booked via the SJ website. Night trains run between Stockholm and the far north (Kiruna, Gällivare), which saves both time and a night's accommodation.
By Bus
Long-distance coaches are significantly cheaper than trains, though slower. Flixbus and Vy are the main operators, covering most major cities and many smaller towns. The coach network is often the only practical option for reaching smaller destinations not served by rail.
By Car
For exploring the countryside, the islands of Bohuslän, or Swedish Lapland, a car is the most flexible option. Roads are generally excellent, fuel is expensive by international standards, and speed cameras are common. In winter, snow tyres are mandatory in many areas. Car rental is available at all major airports and city centres.
By Domestic Flight
SAS and BRA Airlines fly between Stockholm and cities in the far north — Luleå, Umeå, Kiruna — where distances are immense. Prices vary but booking ahead usually pays off.
By Boat
In the Stockholm archipelago and along the Bohuslän coast, ferries and local boats are an essential part of getting around. Waxholmsbolaget operates the archipelago service out of Stockholm; many islands are only accessible by boat.
Hitch-hiking and Cycling
Hitch-hiking is relatively safe but slow and uncommon. Cycling is excellent in southern Sweden, particularly in Skåne and around the lakes of Dalarna, where the terrain is flat and cycle routes are well maintained. Rental bikes and e-bikes are available in most cities.
City Transport
Stockholm, Gothenburg, and Malmö all have integrated public transport (metro, tram, bus). Buy an all-day or multi-day card rather than paying per journey.