If you are looking for places of interest a few worth considering to get you started are:
The museum at Kildonan on South Uist, well presented and very interesting.
The chambered cairn at Barpa Langass (you can crawl into the chamber in the cairn - not for the claustrophobic), near the turn off for Langass Lodge on North Uist, also the nearby stone circle of Pobull Fhinn (Finn's People), there is a nice wee walk between them over Ben Langass.
Near Lochmaddy there is the hut of shadows (a camera obscura) again a nice walk round to it.
On Harris the church at Rodel is well worth a visit as is the beach at Luskintyre and as already suggested a walk up the Clisham.
On Lewis head down to Uig the beaches at Valtos and Ardroil are stunning and you can camp close by, also the Abhainn Dearg distillery at Carnish is well worth a visit better to check that it is open so there will be someone to show you round, if you are lucky it will be Marco (the owner).
If you head to the North end of Bernera there is a reconstruction of an iron age house at Bosta, also the possibility of camping on or near the beach there.
The stones at Callanish have already been suggested but there is also the Broch at Carloway, the blackhouse village at Garenin, the Norse Mill near Shawbost and the blackhouse at Arnol.
As I said just a few to get you started, if you are looking for walks try to get "Walks: Western Isles" by Luke Williams or "Walking the Western Isles" by Mary Welsh if you can't find them anywhere else you might get them in the Tourist Information shops or on the ferry.
Enjoy your trip
Cheers
Donald.
