Certainly! Thanks Zoe! Have you dived in GBR? Any recommendations on the best liveaboard or day dive provider would be appreciated!
I'm not a diver, but for snorkelling at least (and probably for diving too), the best part up there is the Outer Reef. Tours generally leave from Port Douglas (about 1 hour north of Cairns), and if you haven't booked anything I'd recommend staying in Port Douglas rather than Cairns itself (or Palm Cove if you've got a comfortable budget).
For the Reef, book something that's going to the Outer Reef (Agincourt is a pretty common destination), and clarify you're going on a small boat. Note that in this context, "small" means a couple of hundred people - the large boats take a couple of thousand people and often dock at fixed pontoons.
Since you're asking about dive stuff I'll assume you're competent, but it's worth mentioning as well that the Reef here is 20+ kilometres offshore, and most days you won't be able to see land at all. Plus the water is quite deep in places, 15+ metres, so it can be a little unnerving to jump straight into deep water when you can't see land!
Final thing I'll mention is that don't book any close flight connections, particularly through Sydney airport. Because of Covid, a lot of people left the industry and so there's colossal lines for basic stuff like check-in, security, passport control etc. There's stories of people waiting 5+ hours in the security line; that was during Easter peak so it's not as bad now, but most days it's still pretty packed.