We got back a few weeks ago, but I'll throw in that I was pleased with our experience there. CDC certificate was accepted, although we were rarely asked for it. Once or twice; that was it. In our experience, inside dining was well space; there were always vacant tables between parties. We were usually tested when we changed hotels, but our first Airbnb in Lisbon didn't ask for it and neither did an Airbnb in the countryside outside Alcobaca. One guest house required that we purchase a test from a local pharmacy, do the self-test and send photos to the owner along with the pharmacy receipt. The only thing I didn't like about that was it cost us 20 Euro each, but big deal. Just another cost of travel. Another hotel did it on-site for a few Euros.
We had to make an appointment for our COVID test at a pharmacy before coming back. Check your options when you arrive at your last stop. The airports were the most strict on checking docs. Make sure you get there and extra hour (or even two) before you usually would. Or by now, things might be running more smoothly. Depends on your comfort level with leisurely strolling the hallways or running flat out to get to the gate.
We didn't do any biking, but I enjoyed my day with DL Heritage Tours on the Douro. Small group of 5, combination shuttle van and cruise, 2 fun vineyard visits (including walking through the vines at one and a generous tasting an another) and a tasty lunch at one of them. Well worth spending an entire day. I think it would be boring cruising the entire Douro. Once you've seen a bit of it, you've seen it. Finding a tour coming back (or going) on the train might be fun, too. Check the tour descriptions.
As I said, we've been home a few weeks, but I would say that while we were there, I felt more confident in our COVID safety than I do here. Vaccination rates are high and everybody takes their mask-wearing seriously. Masks are a must in hotels, public transportation, supermarkets, museums and the like. Weekends at bars were not cool--pretty much nobody was wearing a mask. We didn't go, but walking through or around them we noticed it because it was so out of the ordinary.
Just go and have fun. There's no reason to stress out about it.