What no to miss in this trip

Have you looked at any guidebooks to get ideas of what the region has to offer? I would start there and perhaps look at some videos or sample itineraries. Rick Steves does a good job at giving some highlights. Another good source is to peruse some trip reports posted here. Check out theses links.

https://www.ricksteves.com/tours/portugal/best-portugal-tour

https://www.ricksteves.com/watch-read-listen/video/tv-show/lisbon

https://community.ricksteves.com/travel-forum/trip-reports/portugal-july-2-18-2025-2-days-with-granddaughter-the-rest-solo

Here's our trip report from 2024 if you think it may be helpful. I would suggest not having a car for Lisbon or Sintra. Parking among other things will be troublesome. No cars allowed in central Sintra.

https://community.ricksteves.com/travel-forum/trip-reports/trip-report-part-1-hills-and-castles-portugal

So you're basically wanting to spend the entire 2 weeks in the general vicinity of Lisbon? That is, no Algarve, no Porto/Douro? (I'd at least recommend Coimbra though).

If I'm being honest, I think you may find the car a nuisance. For the confines of your trip you'd do fine between public transportation and Uber. We hired a car outside Lisbon and wormed our way up North through central and coastal and Douro valley then dropped off the car outside Porto.

I also consider Sintra a day trip from Lisbon, doubt I'd stay overnight.

I think it is a good idea to use Lisbon as a base for a two week trip. Those who like to travel fast and see a few of the famous sites would disagree, but I prefer to take more time in one location and get into the local culture. If you take your time to sniff them out, there are many wonderful semi-hidden gems throughout the Lisbon area.

As mentioned before, a car is more trouble than it is worth in Lisbon and Sintra. My Portuguese family avoids driving in Sintra and downtown Lisbon.
You will probably get much better advice if you provide more information on your interests and goals for this trip.