We used to go to shell island but you need to choose a good spot. I had a rock digging in me one year and the next year woke up at the bottom of a hill, lol. It gets busy as well so sometimes you're stuck with whatever areas are left.
We found this site:-http://www.dyffryn-seaside-estate.co.uk/
It's only 5 minutes from shell island and in my opinion is a better site to stay anyway. Prices may have changed since the last time I went but they charge by the tent. Shell island charge per person so if there's a group of you going it normally works out a lot cheaper staying at dyffryn. The facilities are better, the grass is like a crown green lawn and it's still virtually on the beach apart from the sand dunes. You can camp next to the dunes as well if you want to be away from the crowds and then the dunes act as a bit more cover from the wind. If you want you could go to both but I'll virtually guarantee that you won't go back to shell island after going to dyffryn.
Also you can actually knock the tent pegs into the ground wherever you want to
As previously mentioned, Glan llyn is another good site!