do you think it's possibile to visit the castle and Sigtuna on the same day?
And is the town really worth a visit?
oh great! and how long does the boat take to the castle?
About one hour, but it only runs during tourist season. It will give you a bit less than 2 hours in Skokloster before it returns to Stockholm. But if you want to stay longer you can return by normal public transport.
I think the boat runs until the 12th in august. After that you have to take the bus, but both Sigtuna and Skokloster are pretty easy to reach with train and bus. Bus between Sigtuna and Skokloster however takes around 1,5 h.
Maybe I should say peak tourist season. But in mid-August, most Swedes return to work.