My dad doesn't trust any other places to put his money other than his savings account, which has very little returns. What investment instruments are hard to argue against?

There are quite a bit of alternatives available:
1) SSB
2) Fixed Deposits
3) Short Term Endowments
4) Mid to Perpetual Endowments
5) Switch Bank account to CIMB or Standard Chartered

In terms of persuasion, it's always about finding out his "real" objections about saving money through alternative means such as the above stated.

Some of the possible objections:
1) Other instruments not safe
2) Bank is the only safe place to save money
3) Liquidity
4) Bank's interest rate is enough for me

Persuasion wise, there can be a couple of options:
1) Use an example that is close to his life. For example a cup of Kopi-O. Ask him how much a cup cost 10 years ago versus today. If he had placed the amount in a bank 10 years ago, would he be able to buy a cup today? Or has inflation outstripped the bank's interest.

2) Show him a spreadsheet on the returns of $10,000 with varying returns (0.05%, 0.5%, 1%, 1.5%, 2%, 3%, 4%) over 10 - 15 years. I am assuming that your dad is risk adverse, so 4% would be quite a good yield fot capital guaranteed instruments.

3) Combine the 2 scenarios above and calculate the inflation of his Kopi-O for the past 10 years. Show him the minimum returns he needs to get in order to beat inflation. The crux is to share with him that he is losing money every day by putting it into his bank. Ask him to think about retirement 10 years or even 20 years down the road, how much would he have? Is he okay for his money to continue to Diminish?

Or you may get him to speak to a financial planner like myself!

I'm facing the same issue. Both my parents do not believe in any kind of financial instruments :(