Is Ever really with the time to go and see or spend the extra day in Sintra or Obidos?
I prefer Evora to Obidos as it's less touristy and there is more to see. Ideally, two days would be an ideal time to see Evora and its surroundings.
When are you travelling? Sintra can be very busy in peak season.
Evora has the small Roman ruins and little else if of interest as I remember. Yes, Obidos is touristy, but it's a walled city. You can (carefully, no rails) walk the wall, wander the streets, dine fairly well.
Obidos can be seen in a couple of hours and Sintra can easily be visited as a day trip by train from Praça Rossio in Lisbon.
Doug, I was in all of these towns this past May. Personally I would rank them Sintra, then Evora, then Obidos. Sintra has 4 or 5 excellent palaces/estates/gardens very nearby, easily accessible, each quite different, and each meriting a half a day or more, in my opinion. The town is very nice too, with interesting shops, good restaurants and bakeries, and lots of public outside art, both traditional and modern. I've heard that it is busiest on a Mondays, when many museums in Lisbon are closed. Evora has more than just the small ruins, also a museum, an interesting university you can visit, and a nice main square with cathedral, fountain, cafes and shops. Ancient walls just outside the town. I found Obidos the most touristy, and the main part of the walls sounded precarious to climb on, so I passed on that. Overall though, you can't really go wrong.
I enjoyed the laid-back feel of Evora which is a Unesco World Heritage site, took Rick's suggested walk through the Centro Historico and had the pleasure of having an unforgettable dinner at the 12 seat Botequim da Mouraria. Visited some small - but interesting museums, the Cathedral (views from the roof are amazing), the ruins of the Roman Temple (go back at night when it is illuminated), Sao Joao Evangelista Church, the University. Nearby are the Megaliths dos Almendres, wineries and olive oil producers. ANd most importantly, not many tourists.
I loved Evora for a quick stop but it was very hot ( could not get there before 11:30 and Roman ruins are covered up for repairs.