The local currency is Euros, but some businesses accept Swedish Kroners. It is likely that you can survive your whole trip on plastic (credit card). Cash is genuinely needed only for exceptional things like 5 Kronors (they’ll take a 0.80 Euro if you ask) for the toilet in the Stockholm bus station.
Typical clothing is long pants, a shirt and a handy water-tolerant jacket. Bring an additional light water-proof jacket for real rain and for the boat ride. Bring a bathing suit for a sauna (sauna is a Finnish word) and an after-sauna swim (which is nice no matter the season or weather).
Finland uses standard European round 2-prong plugs. The prongs have slightly smaller diameter than the Brittish/Indian ones (see photo of adapters, click to enlarge). The voltage is 220V. Most laptops etc will work fine with 220V, check your power-supply fine print for “100-240V” or some such. Your computer likely included the adapter in its original packing supplies.
If you want to participate in the workshops, a laptop will be necessary for some of the workshops. Please check the prerequisites for each workshop, so you arrive prepared with all software installed. You can read more about the workshops and prerequisites on the website soon.
There will be wireless access at Käringsund Resort & Conference. For the invited speakers: We will have standard video connection (VGA) and digital (mac) for projection.