As cirrus01 said in summer months most of the accomodation in the islands is fully booked. I wouldn't reccomend going without making a booking in advance. Try
http://www.scillyonline.co.uk for lists of hotels and B&B's. You might have to do a bit of ringing around. Failing that there's the campsite at the Garrison which could probably squeeze you in somewhere although they even get fully booked in summer.
St Mary's is the biggest of the islands and generally has more activity. There are daily boat trips from St Mary's to all the other islands as well as fishing trips, seal spotting, puffin spotting even whale spotting at the right time of year. You can also go to the southernmost pub in Britain (Not before flying of course). And don't miss the pilot gig racing. It's absolutely brilliant. Granted you'll be tossed around in a pleasure boat and get splashed with seawater but the atmosphere is electric. If you go, cheer on Nornour for me
