Make sure you've got twice as much fuel as you think you'll need for a trip round the Island!
You may think you know the distance to your destination, but you're up and down the gears a lot in cars, and especially on the smaller roads in the South/Harris area, you'll use a lot more fuel than expected.
The people here are very religious and observe the sabbath. Be sure you have purchased everything you will need until monday on saturday. All shops on the island are closed on sunday, which includes all food shops and petrol stations. Buses and ferries don't run on Sunday either. Also, expect queues on saturday evenings at petrol stations and supermarkets.
Sports and leisure activities are also discouraged on sunday. There is strictly no fishing or football playing (signs warn you of this). However, it's popular for less religious locals to go to the beautiful beaches and relax :)
Some scottish public holidays are different in Stornoway. For example, some bank holiday mondays are replaced by communion on thursday. This will affect some shop openings, but not banks or ferry crossings.
there are only little narrow roads and you can suddenly be surprised by others on the road - they do wander free.