Yep, definitely the lagoon, going on the scenic train is cool, Muddies - you could spend a morning or afternoon or my kids would love a whole day there, also if you were going to do a day trip, go up to Port Douglas, go to Hartley's Crocodiles....that's great your DH Will get a discount on the rail, will make a huge discount. All, these activities are expensive, Green Island is pretty exxy...