Go EVERYWHERE early. Or go with a pregnant wife and a two year old child. Portugal is wonderful for mothers and their plus-one husbands - always, straight to the front of the queue. Always, from customs to trains.
But go everywhere early. Really. You will see what I mean once you sleep in and arrive at any destination mid-morning.
Don’t waste time with waiting for Pasteis de Belem, your corner coffee shop will have their equal at far less and a fraction of the price… and without an hours wait.
Sintra is lovely. A side trip to Lagos is quite cool, lovely beaches and much less crowded than the ones in Lisbon.
The Tile Museum (link in Portuguese) is fascinating. I will note that I am something of an art nerd.
All the trams are a great was to travel, #28 is not that special and renouned for pick-pockets.
Graffiti and street musicians are ubiquitous. Many really, really good.
Be adventurous with food. There are like 15 or more varieties of bread in every corner café, for a start. The “Time Out Market” in Alfaeda is a little commercial but many famous restaurants have an offshoot there, so you can try places which you might have to book months in advance if you went to the original. Oh, and you can get Paisteis de Belem there.
Finally: you do not need a hired car. Everything is on a train, tram, or bus line. Even without basic Portuguese, it is really easy - and cheap - to get around. I did hire a car when we moved to the coast - which I highly reccomend, there is so much in Portugal outside Lisbon… but obviously time and effort is required.
If you are reasonably fit, then walk. Walk, walk walk.
You will find hidden gems. A puppet shop. A perfect café. A hidden away Doll Museum. The city centre is not huge, and despite dragging among a then 2yr old and a heavily pregnant wife, walking was glorious.
I was lucky to be there on St. Anthony’s day (my namesake) and, wow, Lisbon pulled out all the stops.
Just remember: go early. Relax in the afternoons. Plan everything for mornings, and early mornings.