If you're headed down the coast, you can do the snorkelling day trips out of Townsville (4.5 hours south of Cairns).
Sunsea Cruises do a day trip to the GBR, and also have a glass bottom boat and marine biology presentation on board. Beautiful big boat, MV Reef Cat, has capacity for 200 but they never take more than 130, so plenty of space and not too crowded at the reef. They offer Discover Scuba dives for (I think) $90 but snorkelling gear is included in the day trip price, as is a beautiful buffet lunch.
They have guided snorkelling tours for around 20 bucks, where the guide goes to the spots where they know the cool stuff is (clownfish/anemones) and also explain some of the science of the reef.
Think it's about $170 per adult for the trip but they do discounts if you book more than 10 I think?