I go over a couple of times a year, but I go for the fishing, May and October.
Its worth a trip over in the spring to see.....and hear.... the wildlife. Absolutely stunning sights and sounds. Lots of birds you wouldn't see so readily elsewhere. But apart from that, not huge amounts to see. Go down to Eriskay....ponies and Whisky Galore! Try to drive up at low tide and marvel at the acres of sands each side of Benbecula
Lots of places to stop overnight without upsetting anyone as long as you use some sense, especially on the western side on the machair, but as AA say's buy some stuff there, even if only to hear the locals chatting in Gaelic in the Co-op! If you can, go Mallaig to Lochboisdale...better views of more islands, and a cracking drive to Mallaig. If you have'nt been its worth one visit, but if the weather is crap it's hideous! If the sun is out it's great.