Arlanda Airport is also included in the 72 hours ticket when you use the pendeltåg (normal train).
You just have to pay the airport passage fee of 85 SEK for exiting and entering the airport ( children under 18 pay no fee ).
Remember that the small ferry to Djurgården is also included.
The old trams are also included :
http://www.djurgardslinjen.se/en/service/