Having lived and worked in Shetland for a considerable time and depending on how long you have/transport etc, there is a lovely walk to a place called Fethaland in the north of the island. A link(
www.walkshetland.com ) gives you information on the walk.
I have gone there on a couple of occasions and it really is a fantastic walk and when you get there you will find the remnants of an old fishing village.
I think it says difficult to challenging but if I remember correctly it was fairly easy going. I do remember the farmer who has the land at the start of the walk putting a "bull in field" warning at the start of the walk to deter people from shortcutting through his field but that was a good few years ago now!
Anyway as with the other good ideas posted its worth a wee look,,, oh and a walk across the unique beach linking the mainland to St Ninians Isle and a walk around the island itself is rather pleasant too! Short but sweet!