Just nudging this post as it's now active - e.g. it's Dyffryn Gardens (near Cardiff) opening up for free this weekend, but plenty of others (in Wales).
Give Caerphilly a miss.
Yes there are lots there free when you look. :D
How come Conwy Castle doesn't show up if you click on Conwy in the Open Doors section but you can find it by searching? Is it glitchy? I am guessing on clicking on the area/county doesn't show all available attractions for now.