Heading to Lisbon, Porto, Madeira & Azores in 3 weeks......what's your ONE do not miss sight?

Definitely wear comfortable shoes with good tread as the sidewalks can be slippery especially when wet.

If time allows I’d suggest spending at least an overnight in Sintra.

Sorry but there are too many wonderful sites, in Lisbon and Porto, and can’t narrow it down to just one!

Are you planning to visit Coimbra? If so, do not miss the University tour which includes the magnificent library.

If you’re going to Sao Miguel consider reading Carl’s guidebook. You’ll find more detailed info by clicking on the very first post of the Portugal forum.

Wishing you a great trip!

P.S.
You’ll find a few helpful tips here
Please note that prices may have changed since these tips were from 2019.

Madeira is a beautiful island with quite varied terrain. The levada walks are great; the levadas (small irrigation ditches) follow the contour of the land, so the walks are flat as long as you're following the canals. You can find photos by Googling. If there are folks in your travel party who have an issue with heights, you should seek advice from locals about suitable walking options, lest they turn into a puddle of jelly rather than enjoying the views. You can do the walks alone (I'm sure there's a guidebook available in Madeira), but going on a group walk with transportation provided would avoid the need to figure out bus schedules.

The soil in Madeira is very fertile, so gardens are worthwhile. It has been decades, but I remember the Botanical Garden as being nice.

Probably any bus heading from Funchal to another town would yield good views. The ride to Curral das Freiras is scenic.

The Azores and Madeira and somewhat unique islands, due to the unusual plant life of each. I thought the Azores were more beautiful than Hawaii.
Madeira ia expensive and smaller, you can probably do a short stay there, but Sao Miguel, I recommend three days.

Porto and the Douro River Valley is amazing. We did a week long river cruise on the Douro that was wonderful. If you don't so that, stay 2-3 nights in Porto, (recommend the amazing InPatio Guesthouse BnB, it is amazing. Book early, it only has five rooms).
Then take the train up river and stay in Regua or Pinhao for 2-3 days. The valley is very, very scenic and lots of vineyards for wine tasting.

Lisbon is great, but between Porto and Lisbon, consider visiting Obidos with an ancient Roman wall and Bathila Cathedral.

Lisbon has many amazing sites. One of our favorite days, however, was first visiting Castelo de Sao Jorge and then exploring the Alfama neighborhood, which is right next to the Castelo. Alfama was not destroyed during the earthquake of 1755 so it is Lisbon's oldest neighborhood. Lots of narrow lanes, uphill and downhill walking, and people live here. You will walk by apartments with laundry hanging out side of their windows.

Other favorites are the Gulbenkian Museum, Jeronimos Monastery and the Discoveries Monument, where you can take an elevator to the top for spectacular views.

1) I wore trail runners on my trip to Portugal. They were great on the sidewalks in Lisbon and for light hiking on Madeira.

2) For Lisbon, my favorite thing was the excellent food walk I took with Culinary Backstreets.

https://culinarybackstreets.com/category/cities-category/lisbon/

3) For Madeira, I listed a number of things I enjoyed on this post:
https://community.ricksteves.com/travel-forum/portugal/madiera-without-a-car

Thanks for all the suggestions!

On Sao Miguel, my “must do” is to take a full day horseback riding trip from Quinta da Terca. You ride around the rim of an extinct volcano, then take the horses down to Sete Cidades, ride around the lakes, have lunch lakeside. Amazing! (I have been to Sao Miguel 7 times & this is my all time favorite).
On Madeira, take the cable car in Funchal up to the top of Monte, then take the “toboggan” down. The toboggan is touristy, but is unique and lots of fun.

The sidewalks can be slippery. You're right to be concerned.

Porto: I enjoyed my morning at Taylor's--great self-guided tour and very informative: https://www.taylor.pt/us

Lisbon: really, just walking around and seeing the city was enjoyable for me (Rick Steves has a walking tour); most memorable was the Convento do Carmo. I found it fascinating.