Tram 28 is about the most touristy thing you can do in Lisbon, but here as well as in a few other cases and places around the world, doing such a thing is perfectly enjoyable when done in moderation.
Other such totally acceptable tourist activities include: A Hop-On, Hop-Off bus in Paris; canal boat tours in Amsterdam; celebrity home bus tours in L.A.; and speaking elaborately with your hands for no reason in Rome.
Lisbon's most famous tram takes you through all kinds of neighbourhoods, to experience various lookout points, parks, and professional pickpockets.
You will be amazed that such an old train manages to navigate some impossibly narrow streets, much less not break down completely in a heap of metal and wood. And they come by so often and so cheaply that it is a great way to hop off and back on anytime. It also helps you skip the arduous climb on some of the endless Lisbonian hills.
Estrella offers great park to walk around in for a break, there's a huge flea market on the route, and the Miradorou de Santa Luzia has to be one of the nicest lookout points in Europe.
Miguel from www.id-wear.com - super-nice guy designing really cool T's