We recently went to the aquarium and purchased a voucher off ebay that is 20% off the entry price. (I think I just searched Melbourne Aquarium on ebay.) It only cost me 99 cents for the voucher plus postage. I think it cost about $60 to get in for me and DH (before the discount). We didn't have to pay for DS because he is under 3. I think a family ticket is about $80-90?.

Sorry I can't help you out with accomodation ideas.. It's been a long time since we had a night away