The tram is not as crowded later in the afternoon and easier to get a seat. Do be careful of pickpockets. My husband was warned by a local when he was standing close to a woman with a child, implying that this person was about to pick his pocket. There is even a sign up in the tram itself! Just be aware and you'll be ok!
You are free to hop on and off all trams, if you have the Lisbon Card, as this allows you unlimited travel for the duration of the card.
Tips: The Lisbon Card is very useful especially if you are in Lisbon for a few days. We purchased the 72 hour card and it was so convenient for all forms of transports as well as providing free or discounted entry into attractions. It paid for itself and we ended up marking significant savings overall.