Can't decide between Hong Kong or Japan
I say go somewhere new! Hong Kong is fantastic. I think you could fill out 8 days if you include Macau, the New Territories and even a guided day trip into Shenzhen/Guangzhou. I did that a few years ago - IIRC it's easy to get a package tour visa to go in for the day - the tour company in HK handles it.
Or you could do say 4-5 days in HK and then hop over to Singapore or Bangkok or even Phuket for 2-3 days.