If you're not spending time inside gambling, then one day is enough to see a goodly slice of Macau, though maybe not enough to thoroughly see the entire main area + Taipa + Coloane. I've done long daytrips before, getting the turbojet about 7 am and returning about 7 pm. From the main Macau ferry terminal, I usually get one of the free casino buses to somewhere central like the Grand Lisboa (BTW even if you don't gamble, having a peek inside the original old Lisboa is interesting). From there, with a decent map of Macau you can craft a nice walking tour around all the old Portuguese areas, the fort, etc. See where you are midday and if time, take a local bus down to Coloane village for a look see, maybe even lunch down there. Bus back. Taipa probably has less to see nowadays. The Cotai Strip is really being built up with tons of new casino and hotel/resort properties, of which the Venetian is the most famous. The complex is worth a gawk (the casino itself is actually not that great though, IMO).

Remember that once you get to any casino, all of them have free buses to take you back to either the Cotai Pier (near the Macau airport) or the main Macau Ferry Pier. They need not know that you aren't a hotel guest--they usually just assume you've blown money in their casino. There are many options (ferry or turbocat) to get between HK and Macau all through the day and evening--at least hourly, except that times get more infrequent after around 10 or 11 at night. When you want to leave Macau, I'd just rock up to the ticket window and get the next thing going out. Be advised that the turbos and the ferries end up at different places on the HK side, in case it matters to you. Google and the current timetables should come up online.