leaving me nothing.

I know this is a personal finance sub, but I really have to stop and address this point.

You are not entitled to an inheritance.

Your parents are entitled to die penniless. This is their perrogative.

Acknowledge this. Accept this.

If may be difficult, but you should not be providing financial guidance to your parents if you feel differently regarding an inheritance. It creates a conflict of interest. Hire a Fee Only Fiduciary to discuss and construct a retirement plan.

but is there some no-brainer business or investment opportunity that would make perfect sense for a smart, energetic couple in their "retirement" that could bring in more money?

Start with a budget.

Figure out the expenses needed to be covered.

Understand what your parents expect to spend in retirement.

Once you have that, you can then understand how to allocate the funds.