8 day trip planning-where to go?

Congratulations on your upcoming retirement!

There are so many places in Europe that could fit this description- and in my opinion late May is one of the nicest times of year across the continent. My first thought was Provence- lots of options there and weather will be nice- or other places in the south of France generally. Northern Spain would be another good option. This would also be a good time for areas like Tuscany or Emilia Romagna. In any of these areas you could stay in one of the smaller cities.

I think Zurich/Lucerne will be too cold for what you want. Good luck deciding!

Neither Switzerland nor northern Italy are known for 70 degree Fahrenheit weather from mid-May to late May. Places in Europe that are warmer this time of year are:
Spain (Andalusia)
Italy (Rome, Amalfi Coast and Sicily)
Greece (coast)
Turkey (coast and includes Istanbul)

Congratulations on retiring. I'm retiring on April 10th and we leave for 5 weeks in the UK on April 11th.

I might think about Hawaii or Mexico for warm, relaxing and beautiful.

Congratulations! Boston is a great starting point for direct flights to Europe so that is one simplification - Might not want to spend too long in an (or several) airplanes. London, Paris, Amsterdam, Madrid, Rome all come to mind with direct flights. good food and scenic attractions. All have more history than one can cover in a lifetime, but since you can't do even a little of what they offer that frees you to do a sampler of your choice. None of those have light hikes, but all have the possibility of beautiful walks. All have various tour companies that can take you out on day trips to "nature." The only limitation from the late planning window is that you will pay premium prices for whatever you choose to do... There is always something available at some price. If you are willing to have a second flight added I would add Florence/Siena, Lyon, Andalucia, and possibly Greece.

The northern/central Black Forest is peppered with nice villages, scenic walks, and Michelin-guide dining establishments. I suggest you locate yourselves in Baiersbronn You do not have to plan carefully with a car - just head out on short outings within a 30-mile or so radius and you'll find dozens of nice villages, scenic drives, and walking options.

Baiersbronn dining options - https://guide.michelin.com/en/de/baden-wurttemberg/baiersbronn/restaurants

Baersbronn provides access to one of Europe's great driving routes, the Black Forest High Road (Schwarzwaldhochstraße.)

https://www.black-forest-travel.com/high-road/ (Baiersbronn is just NW of Freudenstadt on the map. Use the links on the left to view the immediate area's small towns.)

This site has some good information for the BF High Road area: https://angiestravelroutes.com/en/black-forest-road-trip/

Additional old-world towns you can explore to the south of Baiersbronn - circle them on a map and create your own route...
Schiltach
Alpirsbach
Wolfach
Gengenbach (photos)
Haslach
Gutach (Black Forest Open-air Museum)
Triberg
Wasserschloß Glatt (moated castle near the town of Sulz)**

I am 78 years old and retired for 15 years. Have done loads of travel.

I NEVER travel to go somewhere to RELAX. I can do that at home. We live near the beach and don't go to Europe for that.
I can't see spending the airfare to Europe to relax and not do the great history.
Still, you can enjoy some scenic places.

Switzerland will be a bit chilly in May, but the scenery is great. Zurich is not so great, but love Lucerne.
The Lake area of Switzerland and N. Italy is great.

If you are interested in exploring villages, consider visiting Tuscany and some of its great places like Siena, Lucca, San Giminancio.

Boston has direct flights to Portugal.