Hi Axel and Sasha,
 
monday we talked about organising different daytrips during the weekend. THe ideas that came up were Birka,
Drottningholms Slott, Gripsholm and Waxholm. Hans-Erik Sätherblom aso proposed Sandhamn and then I said that a
boattour in the arcihpelago might be nice. Anyway, here are some ideas you can propose to the group.
 
http://www.stromma.se/en/STOCKHOLM/Tours/Birka/Daytrip-to-Birka/ A daytrip to Birka (7,5 hours), 310kr per person, which includes
a round trip, a guide on board and on the island and the entrance to the museum.
 
Gripsholms slott, in Mariefred, is open between 10 and 16 every day. The entrance ticket is 80kr. There is an english tour at 1 o'clock that you must book.
http://www.kungahuset.se/royalcourt/royalpalaces/gripsholmcastle/thecastle/visitus/guidedtours.4.396160511584257f21800010267.html
The castle can be reach either by train or by boat.
 
Drottningholms slott is open every day between 10 and 4.30.
http://www.kungahuset.se/royalcourt/royalpalaces/drottningholmpalace/thepalace/visitus/gettinghere.4.396160511584257f21800014785.html (how to get there)
The entrance fee is 80sek and includes a guided tour (at 10, 12, 2, 4 o'clock every day) 
 
Hope this helps
Nanna