If you are going in the late March, you can go to Japan to see sakura, they started to bloosom in the last week of March or early April.
Day 1-5 HK ---> Tokyo
Day 5-10 Tokyo -----> Osaka -----> HK
You can either take bullet train from tokyo to osaka or take domestic plane.
When you are in Osaka, you can also visit Kyoto to see temples!!!
I am sure you will have much fun staying in Japan for 10 days!
Or if you want to go to somewhere adventurous, you could go to Thailand, and then cross the boarder to Cambodia and see Angkor Wat! it is also very good!
Somewhere far, you could choose East Europe, like Czech Republic and the near countries. It is very very cheap to go there. only a few US dollars to stay a night in hostel.