I am only spending two days in Stockholm. In a town with more than 70 museums it is easy to find one which interests you, but there are a few which most people have heard of and would like to visit. A walk in Gamla stan.
Many museums arrange various children s and youth activities in connection with school holidays.
There are boat trips to the archipelago all year, although there are fewer departures. It costs 90-300 kr per person and night. A bed in a dormitory is the cheapest. The changing of the guard takes place every day of the year at the Royal Palace, and sometimes the main guard is mounted. It shouldn't cost more than 350-400 kr.