What's the best way to earn additional income as a civil/public servant other than stock markets?

Doing surveys on survey sites?

As a previous public servant, I'm aware that the rules are pretty strict and no moonlighting is allowed. However, there are some side hustles that can somewhat qualify (but still do take precautions and check through the SOPs/IM) before you decide on doing them.

1. Selling things on carousell
2. Giving private tuition (as a favour, with a small remuneration of course)
3. If you have a car, giving GrabHitch rides (that fall within the rules of LTA - no more than 2 rides per day).
4. Getting a booth in a community flea market to sell pre-loved/new items (our SG equivalent of a garage sale)

There may be more! Look out for answers to similar questions too! :)

Hope this helps!