Liquid investments are investments that could be turned into cash relatively easily and assume various forms, such as savings accounts, Certificates of Deposit, Money Market Accounts, and other interest-bearing accounts offered by banks. Normally, these types of investments are FDIC-insured and although they offer low rates of return, they are relatively less risky.
You could try advertising more, for example, by putting up signs around the neighborhood, posting about it on social media, or having people you know spread the word. You could also try doing it in a different neighborhood that might have friendlier people, or do it in a time and place where there's likely to be a lot of people walking around the area (e.g., near a church before the end of a mass). https://buzzingofferselfhelp.tumblr.com/