Stonehenge

Stonehenge's layout is most prominently aligned with the solstices, mid-summer (June in the Northern Hemisphere) and mid-winter (December in the NH). There are also some (probable) planetary and lunar alignments. As far as I have read, there is not a specific alignment that is special on either of the equinoxes. In any case, there would not be a huge change in sun's appearance from day of equinox to the day or two before or after. The site is absolutely amazing at any time. I highly recommend the Stone Circle Experience, because it allows you access inside the circle and with only a 15 or 20 other people around. We did early morning and it was incredible despite some clouds. Book ahead!
https://www.english-heritage.org.uk/visit/places/stonehenge/things-to-do/stone-circle-access-visits/

There's lots to see at Stonehenge besides the stone circle itself. If you book Inner Circle, your entry will be either in the early morning or the late afternoon; both are outside of the regular entry hours. This means you won't get the audio guide nor get to see the indoor and outdoor museum exhibits, which are really excellent IMO. You also won't be free to walk around the larger property as you have to ride the designated shuttle bus. And the large book shop/gift shop and cafeteria won't be open, AFAIK.

We did the best of both worlds -- went during regular hours the day before our early morning Inner Circle visit. This allowed us to enjoy the audio guide, museum, etc. and to "rehearse" driving there from our hotel in Newbury, which for Inner Circle we had to do in predawn darkness. It was a good thing, because one of the roundabout signs was missing and we could have easily taken a wrong turn.

It seems some people are disappointed that even with an Inner Circle ticket, you are not allowed to touch the stones. Just be aware that is the policy.

Maybe we were just lucky: when we did the Circle Experience in the early morning, they opened the gift shop briefly for us when the shuttle returned us to the visitor centre area. The cafe was also open for take-away coffee/tea and bakery items. Although we did not stay, we were told our Experience tickets were good for the whole day also, for the entire English Heritage part of the site at Stonehenge (which is the only part that requires a ticket) once everything opened for regular hours. Another plus for the early morning--plenty of available parking!
(By the way, we were warned at our hotel to not leave ANY valuables in our car at the Stonehenge parking area.)

We did a mad max tour there, great because we got there just before opening. If you are thinking of driving the traffic was crazy to get there, fyi..

Thank you everyone for your good advice!