I'm not going this year but I've always enjoyed Avebury for the solstice, despite the last few times being wet!
I often arrive by cycling there on my mountain bike along the Ridgeway - I like to imagine how through millenia thousands of others have made a pilgrimage at the same time of the year, and along the same route.
I've found that over the years it got more controlled, and that it gets harder to pitch your tent. The last time I arrived after a six hour ride, I went to pitch my tent in the picnic area and I got turned away.
Anyway it's a great way to pass the longest day. I like to mark my equinoxes and solstices, often by getting of my face, which I'm sure is a practice which goes back to prehistory!