According to this report, Midsummer at Stonehenge seems mainly to be a summer festival like any other, with lots of bands, long lines at the facilities, and shenanigans in the campground. But then, what else could it be? Civilizations have templates that they impose on any sort of celebration, and this is ours for an outdoor party. Although a few people managed to make individual statements:
"This pink lady was standing on top of a caravan doing a lovely dance with her white scarf, and making all of us stop and stare. I have not the slightest clue why she'd do this (yikes, heights!) but I assume it must have been important to her, part of her Solstice celebration."