Any recommendations for investments as an alternative to cash contribution to parents?

If this money is meant to support your parents, then you may wish to keep it liquid. In this case, a large number of options are off the table. A mixture of high yield, no-hurdle savings account such as CIMB fastsaver (protected by SDIC), and money market fund as what Hariz suggested would work out well in terms of averaged returns and diversification.
Take note that while funds in CIMB can be transferred to any bank via FAST instantly, money in money market funds has one working day delay to be transferred to your bank.

Just to make sure I understand your question, the options are

• give 500 cash per mth to your parents
• buy 500 worth of investments either in your name or your parents' name.

Have you asked what they wanted?
I have been thru this phase before, even though I am a better investor than my parents, end of the day I have to respect their wishes on what they would like to do with the money.

Even if they use the money I gave them for donations, or they use it to buy diapers for their grandchildren (not mine).

Otherwise, it's not really like you are giving them money... You become like another CPF board who holds money they can't touch...

How about trying this?
I gave my parents education on SSBs and tell them to consult me if they want to invest. But I am only a consultant with no fee charged. They make the decisions for their money.

Make the decision together in discussion. They could have things they really want to pay for, and not invest.