The 28 tram is THE iconic method of transport in Lisbon and a great way to see some of the major sights of this gorgeous city: the Se, Castelo de Sao Jorge, the Baixa and more.
However, it can get really crowded with tourists during the summer months and around the middle of the day. Many visitors often appear not to realise that this is not a tourist tram specifically for them but an integral part of the city's transport system used by many old grannies and locals with little children etc.
A tip for getting a seat is to take the green metro line (LInha Caravela) from Baixa-Chiado or Rossio to the Martim Moniz stop. From here you can jump on board the 28 tram at the start and be assured of a seat - and a fantastic view, unless there's a giant bottom in your face - all the way through town to the terminus at the other end by the Cemiterio dos Prazeres.
Be sure to give up your seat for a grannie, though!
Martim Moniz metro stop (Green line - Linha Caravela)
Tram No. 28
The Martim Moniz is the final terminal, but half the trams leave from the Largo da Graça terminal (there is a long version and short version of the route). Largo da Graça is near the castle/alfama and with the best miradour bar to enjoy amazing views over lisbon.
Is this your business?