We are arriving in Lisbon from the US the morning of October 31, and neither my wife nor I can sleep on a plane. So we will be quite jet-lagged. I think the most we might be able to manage that day would be one of the three self-guided strolls the Steves guide details (Bairro Alto & Chiado Stroll, Alfama Stroll, and Baixa Stroll). Which of those is the easiest, in terms of both distance and elevation change?
Baixa Stroll is the easiest in terms of elevation gain and walking. The Bairro Alto is easy if you have a transportation pass and are taking the funicular and elevator. All three are pretty tiring. If you are jet lagged and still want to get some sightseeing in I would suggest his trolly tour. I don't have the book in front of me right now but I think there is a number 28 trolly tour. Get on and sit and enjoy the sights. You can get off at the castle stop and have a drink and enjoy the view. Then get back on. Just don't attempt to walk up the hill to the castle as it is a large hill and can be saved for your Alfama Stroll.