I don't really use premade adventures anymore in my games, but HSI is an exception. It is the best sandbox game on the market, all original tasting fantasy, easily slottable into whatever fantasy world you're running.
Your players won't be able to tell exactly what they're facing. Many of the encounters seem like something you already know, but if you take your time to understand what's going on, you'll learn you were wrong, and the real deal is much more nuanced.
The magic items alone are worth the asking price. None of them are what you'd expect, and they weave a fascinating story about the island's history and culture, when uncovered.
Simply exploring the island, hex by hex (with each hex having three layers of secrets to uncover!), is all the gaming plan you'll need for the next year or two. Our office game has been on the island for all of 2019 (ten sessions or more), and they've barely scratched the surface.