Australia would be great! SO and I just got back from there for the second time---we just love it that much! Cairns would be great, except I would go just a bit north to Port Douglas (it's a little less touristy). It's very tropical. You could go diving or snorkeling, tour the rainforest (I highly recommend), go jungle surfing, do a river crocodile tour, all sorts or water activities, lay on the beach and more. There are some BEAUTIFUL places to stay depending on your budget. Also there are some really beautiful islands off the coast you could stay at depending on your budget too. Perhaps you could fly into one for just a night or two (I would recommend lizard island---it would be great for a honeymoon). But there is just so much to do. Oh and the restaurants in port douglas were really good too!
If you have any questions let me know. Also there are some fun places to go further south in australia---just not as tropical.