Thanks to SL's spiffy double decker busses, it's really easy to get to Norrtälje from Stockholm. Bus #676 leave Tekniska Högskolan and Danderyds sjukhus every 15 minutes in the afternoon and return every 30 min in the evening. The trip takes about an hour and there's free WiFi onboard the bus.
The latest bus you can catch and still arrive to the hash on time leaves Tekniska högskolan at 13:35 and Danderyds sjukhus at 13:48. Get off at Norrtälje Gustavslund at 14:40. That is the stop after Campus Roslagen, which is right after the big roundabout with the upside down anchor at the entrance of town. From the bus stop, it's about a 500 m walk to the On Inn with half-arrows marking the way.
Returning to Stockholm, the corresponding bus stop is on the other side of the street but about 250 m closer to the town center. It's a little confusing, but don't worry, there will be half-arrows marking it too. Bus #676 leave Norrtälje Gustavslund every half hour at :23 and :53 until 23:53, but bus #696 continues once an hour after that until 01:37.
You're also welcome to spend the night at the cottage or to pitch a tent in the garden, but please let the hare know ahead of time as space is somewhat limited. (Head? Who said head?) There's also an STF hostel called Hvilan 200 m from the On Inn and Hotel Roslagen just across Stockholmsvägen about a 15 min walk away if you prefer something fancier.
In fact, please let the hare know even if you plan to return in the evening as it would help him plan for enough food & drink. Oh, and also if you're allergic or can't eat something.