Macau is easily reachable in a day trip from Hong Kong by ferry – the ferries leave every 15 minutes and you can check timetables here. If you know when you want to go, I strongly recommend booking in advance, but if you don’t mind waiting a bit or having to book premium category ticket, you can just go to the harbor in Sheung Wan.
For me what makes Macau special is its unique Portuguese architecture and food. It is pretty amazing how so near Hong Kong you can have so different style of architecture. And of course there are the huge casinos, which however are not really my kind of thing – too many people and hassle in an artificial surrounding. But of course they are pretty amazing huge buildings.
So if you have an extra day in Hong Kong, definitely worth taking the trip.