Sigtuna is about 40 km north of Stockholm and 15 km from Arlanda airport, making it an unusually easy day trip from either. From Stockholm, take the Arlanda Express or the commuter train to Märsta, then a local bus (number 570 or 575) to Sigtuna town centre — the total journey takes about 40 minutes. From Arlanda, a taxi or rideshare takes about 15 minutes; bus 579 connects the airport to Sigtuna directly.
By car from Stockholm, take the E4 motorway north to the Sigtuna exit. There is free parking near the town centre. The town itself is easily covered on foot — parking and walking is the standard approach.