Vaxholm is reachable by transit bus from central Stockholm, and an easy (and inexpensive) way to visit a bit of the Swedish archipelago!
There are nice walks to be found in the area, and is worth a visit depending on how much time you have in Stockholm. It is certainly a popular daytrip for the locals!
There are also ferries that will take you there by water (from central Stockholm) if you want to have a relaxing day cruise!
Approx 1 Hour from T-Centralen by public transport