In Cornwall for a week-are the Isles of scilly worth a look for a day trip?

They CERTAINLY are worth a day trip.
We try and go every year and go on the ship from Penzance ( we take the dog) which gives you several hours on the island and an interesting commentary on the way pointing out places of interest like the Minack Theatre and Lands End.
You can also fly from Newquay or helicopter from Penzance.
Superb scenery there and you can also go to the famous gardens on Tresco (arranged for you on the ship if you wish)
The clear waters are almost like the tropics (apart from the temperature!)
If the weather is not too good there is not much apart from the pubs(!) but we have never had bad weather yet on our day trips as you can normally more or less work out what the weather will be.
I don't think you need to pre book normally but perhaps at bank holiday times you may have to. We usually go in June when it is fairly quiet and just before the main holiday time.
We can certainly recommend the islands.

I agree - a great day trip. Obviously flying is more expensive than the boat, but it gives you longer on the islands. If you do take the boat from Penzance, check the different days of the week for timings - I seem to remember that one day each week (Wednesday?) the timings give you longer on the islands.
