Hopping on one of Lisbon’s electric trams is probably the most picturesque way to see the city. And it will save you walking up all those hills the city is famed for. You can just concentrate on enjoying the view.
There are 9 tram lines, but probably the most famous one is the 28E. This takes you right through the city, via all the best vistas. Make sure you get off at Castelo de São Jorge. You can see other routes on the Carris website.
Electric trams are cheap as well. You can pay the driver for a single ticket. But if you plan to take more than one journey, it would be best to get a rechargeable 7 Colinas car. The card itself only costs about €0.50, and then you can charge it with as much or as little as you want. Touch it to the control point as you get on. Recharge at an ATM, post office, or information and sales post.