which summer school program is good

It is hard to answer your question without more information from you.

Do you have any preference in location? e.g. any area near MTR or just HK Island, just Kowloon Tong... etc
What kind of courses are you looking for? e.g. Sports focused, Arts focused, Language focused or some kind of all-in-one program.
What time-frame? Some begin early July to end of July and most begins after mid July till