As an investor, what should I do with un-invested cash?

Just drop it into an ETF and watch it for a while. 

Sometimes the best trade to make is no trade at all. 

Wait until you spy the right opportunity, the discipline that takes will improve your entries in the long run. 

There is always a good buy with stocks. Look more. You could always add to the positions you already have if you have any. If you bought it, you must think it will do well.

Choose a managed fund.