I haven't had a chance to read through this whole thread but wanted to comment quickly since you are leaving so soon. It sounds like you have settled on separate trips to Salisbury and Glastonbury. I think that's a wise decision given your interests. We visited both briefly in June 2007 on our whirlwind UK tour. I wish I'd had a little more time in Glastonbury to tour the abbey; we only stopped so I could walk to the Tor. (Have you by chance read The Mists of Avalon by Marion Zimmer Bradley? It is one of my favorite books of all time.) If you park in the abbey parking lot, you can walk to the Tor from there (we didn't see any "official" parking at the base of the Tor). It is a bit of a hike (maybe a 1/2-hour as I recall) but not overly strenuous.

I do recommend reading Sarum - long yes, plodding no. It was my inspiration for visiting Salisbury. And the tower tour of the cathedral, as someone else mentioned, is excellent. You learn some fascinating architectural history and get incredible photo opportunities and a fabulous view of the town.

I was worried about Stonehenge being overrun by crowds but was pleasantly surprised. The path keeps you away from the stones, but it keeps everyone else away from the stones as well, so some of the atmosphere is maintained no matter how many people are there.

If you're interested, detailed descriptions of our visits to Stonehenge, Salisbury and Glastonbury (and links to photos) can be found on days 4 & 6 of my trip report:
http://www.fodors.com/community/euro...land-wales.cfm