Expensive to get there (don't even think about the boat, unless you have a stomach of iron - you have to fly!) and accommodation is expensive and limited - but after that costs are no different than mainland Cornwall. The expense of getting and staying there means you're paying for empty beaches and lack of crowds.
There really isn't much to do other than explore and have meals - it's a very outdoor holiday - but if the weather is nice, it's utterly stunning. However, if you're unlucky with the weather you may feel it was a very expensive trip for what it was. There are no real indoor options other than dining and one tiny museum.
Be warned, the clear water looks very appealing but is astonishingly cold, even in August / September when it should be at its warmest. It felt much colder than the beaches off mainland Cornwall, for some reason. I was a bit disappointed with that.
Recommend staying on St Mary's for a first trip as the boat connections to off-islands (smaller islands) are good and you can visit them all in the space of a week if you want to. It still feels tiny and cute but there is a little bit more to do, more than one pub / restaurant etc. Plus bike hire, riding stables, small village with multiple shops, museum etc.
Tresco feels different as it is privately owned and operated as a posh holiday resort. Beautiful scenery and the Abbey Gardens are probably the biggest attraction on the entire archipelago but I didn't love the vibe; it was my least favourite island.
You can't be choosy about accommodation if you're going in school holidays. It's more what is available. You're unlikely to find any accommodation on the off-islands in high summer unless you're very lucky and chance upon a cancellation, or put your name down a year in advance and take anything that comes up.