If you are taking metro there, price out how much it would cost to ride share back to DC. It is an utter PITA to get home via public transport from Wolftrap. It's taken me up to 2 hours in years past to come home and I live on the southern end of DC (shuttle bus, to just missed the orange line at west falls church and wait 18 minutes to l'enfant to transfer and wait another 20 minutes for that line). Getting out is fine during the work week due to enhanced trains (well, not now, thanks WMATA) but getting home kills your good music vibe.
You can bring water bottles into the pavillion. Just not food or something that isn't concealed in a water bottle. Any coolers/blankets/etc you can leave on the side but not bring in.
I was just there for Gypsy Kings on Sunday and it was a really lovely experience.
I'm sure the B&S will be great; I'm going too, but I wish it was just them for a solid 2 hours and no one else!