http://www.walkshetland.com/weisdale-and-tactigill-circular.php We walked up the Hill of Sound and across to the cairn on Weisdale Hill before heading down to the Tactigill Burn and following it back to the road. Even if you don't fancy the hill - the views are amazing! - the burn is a lovely walk in itself. From the cairn we could see the east and west coast of mainland Shetland as well as the islands of Foula and Whalsay and north to Ronas Hill and south to Fitful Head. As Elizabeth said, it's not a big island when you see it like that!
I also found Burn Water, a geocache (http://www.geocaching.com/geocache/GC34V2Z_burn-water?guid=96ba6325-08f7-480f-b0dd-f34aaca95a8c ) which our route took us past. The cache is near some clay deposits which were declared unsuitable for working. and have interesting layers in the rock formations.
There are many twists and turns in the burn and we lost count of the number of times we crossed it to get the easiest walk along it. There are numerous pools and waterfalls, most splashing only a short distance but one was large enough to be marked on the map.