If you’re thinking of investing in a new venture ‘on the side’ it might be prudent to keep some or most of your cash in a high interest account, in case the new venture goes belly up, so you’ll have some reserves to pay the mortgage, etc.
But, if you foreswear equities, and as you already have an investment in property, you are left with debt (i.e. bonds) and then asset classes such as commodities, timber, gold, etc. that are generally invested in because of their diversification benefits. But if you don’t already have equities, investing in alternative asset classes is really just a bet on their short term performance. So, assuming you have reasonable long investment horizon, you either go for (a) equities, or (b) bonds, or (c) an equity / bond mix, depending on how you feel on equities and your risk profile. All of which you can invest in either via ETFs or low cost index tracker funds. [Disclaimer: The above is comment / observation and is not a recommendation to follow any particular investment strategy or to buy / not buy any particular fund.]