What would be a recommended instrument for accumulating compound interest?

Perhaps I can rephrase your question to include not just compound interest, but compound returns...

Compounding basically follows the principle of reinvesting your returns or interest to generate more returns or interest. Some investments like CPF can automatically reinvest your returns or interest, while for other asset classes you need to take a more active approach in investing. So I would suggest that this principle of compounding should be more of an approach issue, rather than it simply being one of asset class.

Shares are certainly a good place to start since they yield pretty good returns over time. You can also look at other asset classes that you are more familair with to also diversify your porfolio and risk away. Remember that you should take your risk appetite into account as well.

I would recommend investing in equity.