What should someone, who is 40 years old and has cash on hand, invest in?

There are no straight answers. Numerous studies have shown traditional portfolio allocation to be 110 - your age in equities, and the rest in bonds.

Another is based on your own risk appetite. higher risk appetite, more stocks.

Another which is based on your time horizon. Longer time horizon, more stocks.

We might need to understand what is your current plans...

From there, then we can roughly understand what to advise you.

If you wish to draw down... you might need more fixed income/dividends and less stock for capital drawdown.

If you are still investing aggressively, you might want to consider looking for value stocks.

I have a feeling the core of this question is that 40s is no longer young. Do note that you still have more than 25 years (at least?) of economically productive period.

20 plus years is still a good time for most stocks growth. So I think it's better you continue to invest in stocks. If you don't like stock picking, go for indexes. For SG index, go for STI ETF. If global is your preference (i would go for global being a 40yo) I will aim for IWDA ETF.

ETFs are cheaper in fees and generate higher returns pa compared to mutual funds, unit trusts, insurance policies etc.

A lot of people love properties, nothing wrong. Just note that property returns have declined over the years though so do read up more!