There are some very interesting suggestions here from short to long and varying degrees of difficulty. Over the years I've probably walked most of them and they were all good... And I would confirm that the Kinder walk is definitley challenging - make sure you check the weather forecast before setting out as there is no shelter at all...
Hole of Horcum is impressive and BIG.
The Stipperstones is the source of all sorts of myths and legends - worth cheching out before a visit?
If you do the Gilsland walk see if you can find the tap for spa water in the woods. Smell it even if you are not brave enough to taste it.
Glen Affric must be one of the most beautiful of the Scottish Glens and has the advantage of not getting many tourists. Not part of this walk but if you do go, the Dog Falls walk is lovely. Just a short drive is the Plodda Falls walk with a viewpoint built out over the waterfall.
