The walk from Glen Brittle up to Coir' a' Ghrunnda is not really too big an undertaking. It's about 9 km return, with an ascent of 700m, but it rates as 'stretching' in this guide for two reasons. Firstly, the route-finding in the corrie itself can be tricky, especially in poor visibility. Secondly, a section of simple scrambling is unavoidable on the final approach to the upper corrie. But in good weather the large sandy shored lochan, surrounded by Cuillin peaks and held by a massive barrier of boiler-plate slabs, is easily reached by any competent hillwalker. For the non-climber, this is an excellent trip to a very worthwhile destination.
From the campsite at Glen Brittle, follow the main (Coire Lagan) path that climbs straight up the hill behind the toilet block. At NG420203, take the branch to the right across the burn. From here, the line of the path is a little different from the one marked on the OS map, but it is well made and very easy to follow, all the way into the mouth of the lower part of the corrie at NG446193. There are good views of Loch Brittle, Soay and Rum on the way, and a crossing of the Allt Coire Lagan to add some interest - perhaps lots of interest after heavy rain.
From the steeply rising left turn into the lower corrie onwards, life gets more exciting. The surroundings are suddenly awesome. Massive walls of gabbro on either side funnel towards the lip of the upper corrie. The burn slides down the middle over flat, smooth slabs. The path becomes broken, snaking up scree and crossing boulder fields high on the left side. Watch for cairns marking the way and, if ever in doubt, err on the high (left) side. If you get too low the terrain becomes impassible for the non climber. Ahead all the way is a skyline that might be mistaken for the Cuillin Ridge itself, were it not for the waterfall coming over it. This is the edge of the upper corrie, and it can be overcome easily by scrambling up to the left from the base of of the waterfall.
Pick a safely clear day for this one, and take a picnic to enjoy on the edge of the lochan. Paddle if the day is warm. Lunchtimes don't ever get much better than that.