You can get from island to island very easily and all the islands are very different and worth a visit. There isn't an awful lot to do in the way of organised activities, but plenty of walking, exploring and beach fun.
St Martins is beautiful (St Martins on the Isle hotel is lush) but IoS holidays aren't cheap to be honest. We are going for a week in the summer, but to a campsite on St Mary's as DH is a teacher (in Cornwall) and we get a discount.
Visit Tresco Abbey Gardens if you go, and read The Wreck of the Zanzibar by Michael Morpurgo.
St Martin is one of the quiet ones. Not a great deal to do except enjoy the fantastic beaches, which are utterly fantastic. You'd be better off on St Marys. More to do and good ferry access to the other islands.
I was under the impression it's quite expensive though.