Apart from buying STI ETF with $100 monthly (RSP), what other option worth investing which might give better return over long period (5yrs and above)?

Depending on how much you are willing to invest. For $100 per month, you can look at different types of ETFs (commodities, equities, reits...) in foreign markets or use robo-advisors which can help you customise a diversified portfolio. SSB provides relatively well returns with it being guaranteed, although cannot compare to that of stocks.