Singapore's attractions are man-made rather than natural. The Zoo is amazing, the Bird Park pretty good, but the city doesn't have the wow factor of Hong Kong. There is a lot to see in the outskirts of Hong Kong too, so don't leave it too quickly. I spent a week there and didn't see it all.

I doubt you'd have problems in Cambodia, but like Thailand its likely to be hot and humid in June.

I keep telling people Vietnam is marvellous, but it might be too tiring for a single traveller. You'd love the food though.