I am not sure sustainable food is the only deciding factor for diners to choose a restaurant. I'm writing this based on many popular tourist destinations in SEA and hugely populated countries like Indonesia, China, India & Bangladesh who are unable to implement this in their restaurants.
What my observations have shown is is that 'recycling' is by far the most popular for diners and is practised in many of these countries but, not always throughout the whole country — for example, recycling of used cooking oils to make bio-fuels or even to make soap. Good sewage treatment plants where they use enzymes to treat the waste, giving an end product that is odourless, are important too.
Today there is a great push to stop using plastic straws, and many restaurants/hotels etc. are using bamboo straws instead which can be washed and reused. This also helps support the local industry as part of their corporate social responsibility. Plastic bottles, cans and bottles are being recycled, and one key way to do this through is to motivate your staff by giving them proceeds from this as a reward.
Solar energy is growing, and usage of solar energy in restaurants will help reduce the carbon emissions. Non-smoking restaurants are also becoming more common, which is something the health ministry of many countries pushing for.
In my opinion, the above factors, if they can be practised, will be a big step towards sustainability. To start somewhere will only enhance the efforts for sustainability. It's also important that by doing this, restaurants are proud of their achievements and thus their contribution towards society and mankind.
Check out 6 Steps to Increase Your Restaurant Sustainability