A great one for kids that are train fans. You can stop off half way and take a walk to a fantastic waterfall.
Castles and dragons, dungeons and ice cream.
If you're in Wales, near Machynlleth or Dolgellau, then King Arthur's Labyrinth is an interesting experience. Not a whole day out with the kids, but something different...though little ones ...
If you go down in the woods today... you may see Dr. Who and Merlin!
The Ironbridge Gorge Walking Festival runs each year with a number of different (FREE) walks around the Ironbridge Gorge Area.