Comviq cash card (i dont know if it's the best, but it's very common).
Costs 250 sek and you can call/surf for the hole amount.
A call costs 0.69 sek opening fee and then 1,99 sek/min.
And of course you can refill the same card.
A common way used to be to call, but hang up before they answered,
the person on the other end knows what it's all about and calls back.
HA, oki see you in Mariefred / Åkers Styckebruk on the 22 July then