Lisbon is a terrific city. It is also a hilly city, which makes it a great place to start warming up those legs for walking. Years ago, when I lived in Lisbon for an academic year I put together a “walking tour” for friends who came to visit while I was working. Since then I have posted it on the forum, and I think it gives takes you through the main central Lisbon sights. Non-stop walking at a good pace, it’s about a two hour route, but you could spend days if you stop to see the places.
https://www.caminodesantiago.me/community/threads/my-walking-tour-of-lisbon.7257/#post-605793
This doesn’t include the Tile Museum (which you pass on the caminho out of Lisbon), or the Parque das Naçoes, which is also on the Caminho. Nor does it include Belém, where the monastery and the coach museum are, as well as the Torre de Belém and the monument to the discoverers. BUT... for a day’s intro to Lisbon, I think it’s a pretty good start.