Get a boat from central Stockholm out to the island of Vaxholm (about an hour away). Vaxholm has cafés, shops, and an old fort museum that you can go and visit. It's a great day trip for a sunny day. You get to see a lot of Stockholm from the water on your way out.
Waxholmsbolaget also runs boats to loads of other islands - their website has good English pages telling you about them.
The ferries go from Stromkajen near the Grand Hotel. Look for the Waxholmsbolaget sign.
www.vaxholm.se/turism/eng/index.asp
www.waxholmsbolaget.se/
Updates
Forgot to say, try the "Silltruten" cafe in Vaxholm, which has great sandwiches and amazing cakes and pies. If you like stuff for the house, follow the main street round opposite the fort and keep going and you'll find the well-known Magasinet shop. There are also a lot of shops that sell boating stuff and outdoor clothing and so on.
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