I went to the transport office and got a map of all the local tram routes - we hung it on the wall, and my child and I rode every tram from one end to the other.
Took a few weeks to ride all the trams in Helsinki, and it got a bit boring towards the end as several tram routes terminated in the same location. But every tram we'd get on in the middle, ride to one end of the line and go out for a walk, then ride to the other end.
Recently I suggested we do it again, as the trams have been renumbered a little, and there are two new lines available but he's lost interest. Shame, but doing the original routes was a lot of fun and I still have the route map on my wall along with the star-stickers we placed on it to mark the route numbers we'd completed!