It is very wise to buy Stockholm card, you may buy from 1 day to a week. One day or 24 hours pass is a ticket on which you would have a chance to visit too many city attractions and around 70 museums for free. 

Free public transport and many more things either for free or on special discount. Most of the museum and attractions are open from 10.00 am to 5.00 pm daily; use the pass wisely so you would have a chance to use this card for two day (24 hours.) 

Children from age 6-15 would get 50% discount on the Stockholm Card.