First, it's called Macau, or Macao in the Portuguese.

Be prepared for everything in Macau geared towards Mainland Chinese. I hadn't visited for a few years and noted how much it has changed. The ferries from Hong Kong to Macau are busy with Mainland groups, so go for the higher class (I'm sure you would anyway) or, best of all, go via helicopter.

Racing in Hong Kong stops for a few weeks in summer, but if you are here on a Wed (night meeting) or Sat/Sun (day meeting) consider going to the races. Happy Valley on a Wednesday evening is an experience.

BTW, Hong Kong + summer = hot, humid, rain/typhoons, so just be prepared for that. If a typhoon is coming in, ferries, flights, etc. can be cancelled. And Macau is usually hit worse than Hong Kong.

Anything specific you are wondering about?