Comfy shoes for walking around, pavements can be slippy.
Sintra is fantastic - really worth the day trip. We got the bus up to Peña palace, saw inside it, did a short walk in the park, walked down to the moorish castle, followed a trail near the entrance towards the centre, grabbed some lunch and then headed to the Quinta da Regaleira. There are other things to see and do, but that worked for us and it was an amazing day out.
LX and Timeout definitely worth a visit.
The castle worth a wander round. But buy tickets for there (or any of the other sights) online - you can do this just on the day you go, it saves queuing.
The train was a cheap and easy way to get into the city from the airport.
YY to the original pastel place, the queue moves fast.
Drink wine, wander, relax.