First-timers to Portugal, overwhelmed, please help!

Hi
Some info you may find helpful.
In Lisbon an Porto I use the Yellow bus Hop on Hop off tourist bus - I find it to be a easy way to see the major tourist sites in the city without getting lost or worrying about local transport ( just make sure you don't miss the last bus).
https://www.yellowbustours.com/en-GB/Lisbon/Circuits.aspx
Belem you can use a full day to see all of it's sites: Torre de Belem, Padrao dos Descobrimentos, Jeronimos monastery including the inner cloister, Presidential Palace, Coach museum, Museu da Marinha.
For local ( Lisbon) public transport I use Viva Viagem card for the systems that participate in it: https://www.metrolisboa.pt/en/buy/viva-viagem-card/
I also use taxis or pay at the bus / train station for other systems not on the card.
Train from Lisbon to Porto I use their bullet train called Alfa Pendular.
https://www.cp.pt/passageiros/en/how-to-travel/alfa
Why Viseu? Is it for the Dao wines (strong, full bodied) or Viriato - the warrior that led the Lusitano people against the Romans?
Other options from Porto are Guimaraes ( where Portugal was born) or Braga ( Bom Jesus de Braga or Sameiro), they both have Yellow bus.
If you want to stop between Porto and Lisbon I sugest two options:
Aveiro - it has canals and a lagoon or Coimbra - it has one of the oldest universities in the world.
Cascais is a nice place with a small beach and a slower pace than Lisbon.
Since it's your first time to Portugal be aware of two things: pickpockets in crowded areas and in restaurants all the items that the waiter brings to the table have to be paid for and are not part of the cost of your plate unless if the restaurant offers a complete meal deal.
Happy travels

Lisbon

If you like museums, I highly recommend the Gulbenkian Museum. It's one of the highlights of Lisbon for us. This is an amazing private collection that consists of the Founders Collection and Modern Collection. I am not a fan of modern art, but we really, really enjoyed the Modern Collection, and learned a lot about Portugal from their exhibit that's on the lower level in the Modern Collection building.

We did not go to the top of Belem Tower. Instead, we visited the Discoveries Monument, and took the elevator to the top for some spectacular views. There were no lines for the elevator. Belem Tower always has very long lines.

Our favorite day was the day we visited Sao Jorge Castle and then explored the Alfama neighborhood. You will hear Alfama is touristy. That is true only near the castle. A short distance from the castle it is not touristy at all. We passed apartments with clothes hanging from clotheslines, balconies, heard radios and tv coming from open windows, saw children playing in the streets, etc. Streets are very narrow with lots of uphill and downhill walking. We saw very few tourists. We sort of followed Rick Steves walking tour of the Alfama; also got lost but that was fun.

If you get tired of walking, don't hesitate to take taxis. They are very inexpensive and plentiful.

If you like fresh fish and shellfish, I highly recommend Cervejaria Ramiro, recommend by the late Anthony Bourdain.

We went with a small tour group to Sintra, and we really like the Pena Palace. The exterior is amazing!!!

Haven't been anywhere else yet in Portugal.
Hope this helps.

Have you tried calling the airline to get a better return flight? Something similar happened to us. After speaking with the airline (TAP) I found a flight on a different day with better hours. There was no charge for changing our flight.

Regarding Porto, the Douro Valley,

You posted:17: train to Porto
18,19, 20: explore town, wine tour, bus to Viseu for a daytrip
21: leave for?? Would Cascais work?

We loved Porto and had a day there with our tour that included a 7 night Douro River cruise. The cruise was fabulous, but probably not your choice. Still, you could take the train that runs on the north side of the river to Regua, perhaps spend the night there.

You can explore Porto on your own. It is hilly and a bit strenuous, but not too bad. There are some nice restaurants along the waterfront.

Also, we loved the Bed and Breakfast called the InPatio Guesthouse in Porto. It is fantastic. Book early, it only has five rooms.
We also did a day trip tour to see Santiago de Compostela north of Porto, in Spain. It was great.
Cascais is nice. Sintra is good for a day trip from Lisbon. Obidos worth a couple of hours, it is a small quaint town with a Roman Wall. Batliha Cathedral is nice.

Thanks, everyone for the great suggestions. Much appreciated!

Since you'll have your luggage with you, you may want to spend your last evening in Lisbon or within about a 30 minute drive to the airport (taxis aren't expensive, Uber is cheaper), so you have a place to leave your luggage until departure for the airport around 10-11 pm. That could be Sintra or Cascais. Sintra is better as an overnight than a day trip, so maybe stay there on the 21st and visit Cascais as a day trip while you're in Lisbon.

If there's luggage storage available - maybe at Oriente Station in Lisbon? - then you could stay most anywhere on the 21st (Coimbra is nice for a day/night, on the AP train route, get to LIsbon in the morning to store your things, then go to Cascais for the day.

Are you still going?
We (couple in their 60's who share your "likes") too have booked a May trip to Portugal (but we fly from the US to and from Barcelona, then take Ryanair and or Easyjet between Porto and Lisbon).
Our plan has us arriving in Porto on May 4 and leaving Lisbon May 20 with much of our time (9 nights) spread around the interior of the country.
Hope you are coping with the uncertainty we are all feeling. I am certainly hoping all the flights , etc. are working enough by May!!

We don’t know if we will be going or not. Trying not to stress out over it.
Hoping for the best, for everyone.

Honestly, Id cancel....for your health.

We did cancel, of course. Maybe some day.