Chantal has given very good advice.
<<"maximum scenic beauty...">>
Wengen and the Jungfrau region are spectacular. In my opinion, that is the most wonderful High Alpine experience in Switzerland...and a probably the high point of all Swiss scenery for a visitor. Lots to see and do, even if you do not take the exorbitantly expensive trip to Jungfraujoch.
Please keep in mind that late March is still pretty much Winter up there, so many hikes at higher altitudes, may not be accessible. That includes those seen in the attached photos.
However, seeing this comment -
<<"walking in towns, UNESCO sites, architecture, art, and history is what we like.">.
suggests other options to me.
One of the benefits of traveling in Switzerland ( or a disadvantage, because there are too many) is the number of options and choices. So, I'll mention some other options, which won't necessarily fit a simple travel schedule. ;-)
Zürich and Winterthur have a few of the finest art museums in Switzerland.
Basel is known for its art museums.
In Central Switzerland, Luzern stands out.
For example- The Rosengart Collection in Luzern, 3 minutes from the main station, has one of the best collections of Paul Klee in Europe, as well as superlative Picassos. Luzern has a quite nice old town, if you can elbow your way through the tourists. In late March, that won't be a problem.
There is quite nice, quite small Picasso museum in Luzern Old Town.. Maybe the correct term is "was" - I can't find reference to it on the internet. Oh well - here are a few more:
One of the nicest old towns in Switzerland, because of the UNESCO World Heritage Abbey and Abbey Library, is in St. Gall. (Sankt Gallen).
en.wikipedia.org/wiki/ Abbey_library_of_Saint _Gall
A lot fewer tourists than Luzern, but, quite a bit out of the way.
And, the old town in Bern is quite nice to wander in, and very accessible as Chantal noted.
The Old Town in Bern is a Unesco World Heritage site.That's a start.