Yeah, forgot about the 'fika' - but maybe they´ll get enough in Göteborg. And the
boat ride to Vaxholm is another thing that might be worth doing - actually I may be doing this tour myself later this summer (I used to live in Stockholm so I have seen it all, but my wife and kids have never really seen much there). Tickets are still reasonable: 70:- one-way for adults, 45:- for 7-19 year olds.
Another good thing to consider is to get the
Stockholm Card that gives you free entry to 75 museums and attractions, free travel by public transport, free sightseeing by boat as well as several other bonus offers.