Done Barra to Lewis at the start of June, can't help about hire cars as we took our car, was there for a week and done pretty much all we had wanted to.
Was in a b&b in Castlebay which was pretty central, would say a must do is see the plane landing on the beach, lovely beach on Vatersay beside the community hall, we seen all we wanted to see of Barra and Vatersay by car in a day
Had another b&b in Daliburgh, South Uist, because we had the dog with us we eat in the dog friendly Borrodale Hotel in Daliburgh where the food was lovely, was lovely touring about the Uist's, Berneray, Eriskay and Benbecula, loads of stops for lovely views/beaches
The drive for the Leverburgh ferry up the west coast of Harris Is cracking, beautiful beach just out of the village and the view further up towards Seilbost and Luskentyre is amazing and a walk along Luskentyre is a must do, on Lewis we did the Calanish stones and Blackhouse at gearrannan and headed up to the Butt of Lewis lighthouse which was lovely but bloody windy, the Iolaire monument was also a must visit for me
There is an Outer Hebrides thread on the second page of the general travel area with some good advice