You'll definitely get your exercise in hilly Lisbon. There are numerous viewpoints down once you get up to them. I suggest an orientation walking tour the first day (maybe even the first afternoon if you are arriving jet lagged and want to stay awake - walking helps me do that!). I took a free walking tour (you tip at the end if it was good) that lasted about three hours - very worthwhile.
You could hike at Sintra if you want to. Most people arrive in the town by train and take a bus or taxi up the hill to the Pena Palace and the Moorish castle ruins. But you can hike up and/or down instead if you want your exercise. However, you would use up some of your time - and it can take a lot of time to get between places and still fit in everything you want to see in a single day in Sintra. (Hint: buy tickets online ahead of time for the Pena Palace in Sintra or expect to wait in very long lines once you get there!)
Many others would day trip out to Belem and/or Cascais on the coast (on the same train line). Most people consider the monastery at Belem a "must see" but I skipped it because, like you, I'm not really a museum person. I did go out there to see the Monument to the Discoveries out on the river, though, before continuing on the train out to Cascais, a touristy but lovely beach resort town.
