I used Nycoping as a base purely for financial reasons.
The local bus stop is a couple of hundred yards to the left as you exit the Airport, by the time i had smoked a cigarette the bus arrived, i bought a 100Sek (£9) card using my debit card from the driver (each journey costs 22Sek), the journey to Nycoping bus terminal took about 15 mins, there is a cash card machine right outside the bus station no fee.
I stayed at the Ibis on the outskirts of town £92 for 2 nights, i caught the train, an easy 10 walk from the bus station in the morning, i bought my train tickets on line, a return to Stockholm £12.42 and a return to Norrkoping £9.85, about 1 month previously.
As i only had the one day in Stockholm i bought a ticket for the under the bridges boat tour £17, the trip takes about 1.50hrs and it was one of my best decions, it was October and it was definitely getting a bit nippy walking around Stockholm, the guards outside the palace looked misreable and bloody freezing he he, the next day i jumped onto the train to Norrkoping, if you walk down to your immediate right you will see a small shop where you can buy another 100Sek ticket for the bus to Soderkoping, the busstation is just past this on the right.
The 100Sek will cover your return journey, and a tram journey, from Soder tull back to the train station.
When you get to Soderkoping and cross the canal there is a nature trail and you will find yourself in the middle of a forest, what a contrast to Stockholm central train station jeez i could live without going there again.
When you have walked for a few hours a kebab from the pizza place in the middle of Soderkoping is spot on and good value.
Well there you go, an alternative to staying in an expensive hotel in Stockholm for 2 days, and i have earned £40 of my next stay at an Accor hotel.