My travel plans have me unavoidably driving past Salisbury about 6:00 pm, and I'd love to make a side trip to Stonehenge but it's too late. (the park stays open until 7:00, but the web site says they stop letting people in at 5:00.) Is it possible to look at the stones without an admission?
I do not believe so. You need a ticket which includes a specific queing time.
Yes, you can https://thirdeyetraveller.com/how-to-visit-stonehenge-for-free/
When are you going though? It'll be pointless if it's going to be dark at 6:00 pm.
I passed Stonehenge last week on the way back home from Devon whilst driving on the A303, which passes within a few hundred feet. You can’t stop on the A303 itself, as it’s single carriage, but there is a farm track where you could pull in and be underwhelmed as to how small they are! Avebury gets you a much closer look at standing stones.
I already know about that! I've seen Spinal Tap.
The Stones are imposing if you are standing next to one; but I don't know how close they let you get today. You can buy a dual ticket for Salisbury Cathedral and Stonehenge. Buying at the Cathedral lets you see that and get your timed ticket for Stonehenge. Avebury is also worth walking the circle. There are several Monoliths and Paleolithic sites throughout Great Britain and Ireland. The Orkney Islands are said to have one of the best sites.