Callanish stones and Carloway Broch on the West coast are well worth a visit.
There aren't that many footpaths, a few estate tracks, but Scottish access laws mean you can go pretty much where you want. I warn you it is either pretty boggy or rocky or both. There's a nice walk down a track near the West coast towards Loch Hamnaway; it starts near the end of the Uig road. I think there are some new paths further up the coast near Carloway; But you can just find a nice bit and set off, but the going can be quite rough. Great Bernera is worth a visit.
Not sure if you are bringing a car, there are a few buses but not as many as there were since council cuts. A lot of visitors cycle. Hitch hikers usually get a lift quite quickly.
There are hotels, b&b's and bunkhouses but things can get booked up especially in July and August.
Midges - yes lots.
Getting there: flights from Glasgow to Stornoway but they are quite expensive. Ferry from Ullapool to Stornoway or Uig in Skye to Tarbert in Harris. If you are bringing a car you do tend to need to book in the summer as the ferries get very busy. However, just because they say they are full doesn't mean they are: I am usually on standby and nearly always get on.
Nothing is open on Sunday, no buses, very few taxis.
If you like hills, head down towards Harris. If you have time to go south cross over on the ferry from Leverburgh to Berneray and have a look at Uist as well, the ferry journey itself is worth doing. the scenery changes a lot; Lewis is mostly moorland with some nice rocky coastline, Harris is hill and rock with some spectacular walks, Uist has the machair which is all sand and flowers and pretty beaches.
Negatives: It rains a lot and the wind rarely drops completely. And when it rains there is nothing to do; The Islands only cater for tourists who like the great outdoors. And shops, once you are away from Stornoway are a bit basic. And, apart from a big Co-op there's nothing much in Stornoway either.