Since Stonehenge is getting upgraded, have you thought about a miniature railway going around the stones? Have the driver dressed like a druid.
Also, considering that folks can't get close to the stones, another idea I had was a horizontal bungee jump. Folks tie a rubber cable to their waists and they get flung over the top of the stones to the other side. Then they'll have about roughly 2 seconds to take some photos en-route.
Maybe special culinary events where folks pay top $s to eat a three course meal on the big rock inside the stones. The meal is prepared by a 5-star chef using animal sacrifices conducted in front of the diners. Obviously he has to wear a druid costume too.
Oh, and since Doctor Who was filmed around there, why not have models of famous Doctor Who enemies scattered around the site? That'll bring in the paying fanbois. It could gradually evolve into DoctorWhoWorld and you could sell vibrating screwdrivers at the new visitor centre. Along with a Dalek. Maybe a Dalek dressed in a druid costume.
Here's another. Let people pay to be a Druid-For-A-Day where they get to dress up and conduct their own pagan ceremonies in full public view. Of course, you'd have to draw the line at sacrificing virgins but given that there aren't any teenage virgins in the UK any more that might not be a problem.
Feel free to use any of these but if you do, I want a big brass plate with my name on it erected near the site.