Thailand has several good NGOs (e.g UN related) on environmental issues. Are you wedded to staying in HK right now or would you consider working in the region, getting more exposure, and then transfering back?
Other ideas
- consider research/academic career, look for RA position. Consider a PhD if you really fancy academia.
- consider similar/related career like town planning. Will need further qualification.
-Check out the Civic Exchange. Considering volunteering there if they have no paid positions open
- secondary school teaching - range of subjects possible, ranging from ecology or geography, to general knowledge
- consider going into something that builds on your masters but better tapped into the dominant industries in HK. E.g. would you consider joining finance, as an ESG analyst, green bond specialist, equity analyst specialising in sectors relating to the environment, private equity fund targeting similar themes etc etc. You'd have to do further training, and you may hate the idea of finance, just throwing broader ideas out there for you to consider.
Good luck - it's an exciting time, and you can afford to explore different careers and take risk, at this early stage!