I don’t know where you’re traveling from; Arlington, Va… is what’s shown in your profile.
Thailand would be my choice: still inexpensive, great weather during your travel dates, urban, mountain and seaside venues to explore.
Hong Kong, Beijing, Shanghai, Singapore and similar cities are fabulous but Thailand offers more opportunities for a diverse vacation. I lived in Southeast Asia (Malaysia) and my first choice after visiting all the cities you’ve mentioned is for a return vacation to Thailand.
Other wonderful alternatives are Vietnam/Cambodia and Vietnam/Myanmar.
If you do decide on something other than Mainland China, your excursions can be booked once you’ve arrived in your chosen city.
If you’re the worrying type this can be stressful, but please believe me, once in Thailand you can get the same tours/airfares offered in the US for less than one-third the price,
No matter where you go, you’ll have a wonderful time. Good luck to you!