Try to get a seat by the window to enjoy the views - if the tram looks too busy, wait for the next one to make the most of the trip. The old yellow trams have been riding the hills since before the Second World War, and No. 28 follows a perfect route for sightseeing - you can take it up to the Castle and The Sé (Lisbon's cathedral) to name just two sights along the route. It also passes through many of the main districts of Lisbon, but the best part is when the tram starts climbing to the Alfama. If you lean out of the window here, you can almost reach into people's houses - it's THAT close to the walls of their houses! I wouldn't recommend this though in case you lose a limb!
You can get out to explore the Sé, or head to the Miradouro de Graça which boasts fantastic views of the city. The last stretch goes through more narrow streets and hairpin bends up to Graça, where you can explore the Igreja de Sao Vicente de Fora. Tram 28 can also take you to Feira da Ladra, the city's flea market. The best bargains are found here, and everyone is encouraged to haggle the prices. It takes place on Tuesdays and Saturdays from 6am-5pm. The first tram leaves at 6am, and it runs approximately every ten minutes until around 11pm. Not to be missed if you're visiting Lisbon.