Drystone walls do not just exist in the UK. They're very common in Ireland for starters. And on that note, a previous poster suggested Blarney Castle, which isn't in the UK. 
With regards to the right to roam, someone mentioned not being sure of how N Ireland and Wales compare with England and Scotland. There is no right to roam in N Ireland, it's very restricted. Not sure about Wales.
Now, to the question being asked!
My children's favourite UK places have been
HMS Belfast
London in general
Ironbridge museums, particularly Blists Hill
York, especially the Shambles and the Yorvik museum
Windsor Castle
Bekonscot Miniature village
Titanic museum Belfast
Caernarfon Castle
And whilst children aren't generally huge fans of scenery and the like, even they enjoyed the walk up to Symonds Yat and the view over the Wye valley