Optimist is correct about some of the GBR - some of it is quite a way from the coast, some of it is closer to the coast. Directly from Cairns it is quite a way out, but just an hours drive north of Cairns from Port Douglas, it's a lot closer to the coast. If you are near Cairns, the cheaper GBR operators will take you straight out to the reef making it a long boat trip, quite a few of the more expensive ones will drive you up the coast and then take you out on a shorter boat trip. That's one of the advantages of Herron Island - it is actually on the reef, long boat trip out, BUT you stay out there and you can snorkle off the beach directly onto the reef, plus the dive spots are very short boat rides around the island.
Optimist is also correct that you can find similar tropical reefs in Fiji. The Great Barrier Reef is famous because it is the largest coral reef in the world, but any healthy tropical coral reef is spectacular if taken to the right spot.
Another thing to consider is airfares, particularly if you are using airline miles - you might not find your airline miles go to Fiji and Australia, most airlines go direct US-Australia (or via New Zealand), you might find trying to include Fiji makes it a very expensive flight. You'd think it would be easy having a stop-over half way, but because of the limited demand, it isn't easy.