I have just returned from two weeks in the Outer Hebrides although I went to Lewis first and then on down to the Uists.
If you sit in your car in the car park outside the visitor centre at Balranald early morning or late evening you should be able to see a Corncrake at fairly close range. If you drive out to the car park at the far end of this reserve you should be able to get very close to singing Corn Buntings on the way.
All three Divers are very easy to see as they seem to be off every beach in the Uists, especially Great Northerns. Try the bridge from Lewis over to Great Bernera, I saw a family party of Red-throated Divers on the Loch.
I was too early for the main Skua passage but saw plenty of Bonxies on Lewis.
Birds like Black Guillemot and Rock Dove are very easy to see and there are Puffins on the sea once you leave Uig Harbour. The Butt of Lewis is good for nesting Fulmars and the nearby reserve at Loch Stiapavat has a small hide-Corncrakes there as well.
Have a great trip. The only thing that spoils the Outer Hebrides is that where ever you go you seem never to be very far from abandoned vehicles ranging from large lorries, coaches, JCBs, tractors down to cars, all rusting away in fantastic scenery. Abandoned houses are also just left to fall down.
Roger