Best option this late, which would be my first choice anyway, would be to stick your tent in your pack and head into the hills and wild camp! Pitch somewhere inconspicuous, arrive late and leave early.
Same as above - Pentre Bach. Its a bit off the beaten track with no big signs, easy to miss etc. The bigger site further down the valley towards Beddgelert at Lyn Cwellyn http://www.snowdoninn.co.uk/camping_site.html
is also a bit out of the way and seems not to get too full. No idea what they will be like this weekend, mind!
Well I'm heading South after aborting a weekend on Skye, and am heading for Capel Curig. Not familiar with Wales, so my pals have chosen the site, somewhere near Tryfan....... Doubt we'll be quiet though!