Hanoi could be good too. My kids loved it, even Ho Chi Minh (though a little bit further). We did Taipei as well, and also Guilin / Yangshuo is a little it over an hour flight. Don't hear good things about Sanya. We went to Manila twice as well, but have friends there. We did 4D/3N Bangkok with the children (still love it), KL is feasible just like Singapore (although a little further again). Otherwise, Shanghai. Especially with a morning flight and later afternoon / early evening flight back into Hong Kong. My kids do fine in cities, we'll pick hotels with swimming pools so it doesn't have to be a resort (mine are now 6 - almost 7 and 4).