I thought I would post about our experience last month traveling in Andalucia. This was a trip that had been postponed several times over the last two years due to the pandemic and now, fully vaccinated for some months, it finally seemed like we could do it. Originally the itinerary combined Portugal and Spain, but when the Portugal part fell through, we focused on Andalucia instead, and that turned out to be a wise choice given no border crossings or extra requirements to be concerned with. In a nutshell, we flew non-stop to Madrid, took a train to Cordoba, and did a self-guided bicycle tour followed by two birding tours (just us and one guide) out in the countryside. There were mask requirements indoors, and in some places outdoors; I did not see anybody raising a fuss or not complying (unlike where I live). We felt safe everywhere, the only locale where it was a little iffy was the Alhambra in Granada, but even when indoors there people were respectful of spacing and wore their masks. One bus ride (Granada to Ronda via Marbella) was a bit crowded but again folks were masked up. Nearly all dining was outdoors, which in late summer and the late dining hours in the evening, was just lovely.
When we flew over the guidance was still that any American could enter, but after we arrived that changed to vaccinated+negative test. Masks were required on the airplane (again, no fuss, on Iberia). We used the Spain Travel Health app to complete a required form before we entered, and an app called Radar Covid that would help with contact tracing presumably (although it relies on people reporting when they're diagnosed, which a Spanish friend told us was probably not likely). To return home we had to have a negative antigen test; I'd booked the tests at the lab at the Madrid airport for the day before we flew home, and the lab had the results to us before we even got to our hotel; I could see doing it same-day next time. All in all the trip was a success, and I have to say I felt safer there, even with the flights and trains/buses, than I do traveling anywhere near me in the US. Honestly unless it is to see family there is nowhere in the US I would want to visit right now. The situation overseas changes weekly, we actually didn't know if we'd get there until a few days ahead of time, but it was so worth it. My main advice would be to consider your itinerary, try to go to places less-traveled and avoid crowds. That's our general rule anyway and it certainly works well for these times.