One possibility is to setup one or more shopping cart websites selling something. eBooks and software (already mentioned) require no inventory or shipping. A drop-ship arrangement with the manufacturer is another option. Otherwise the right balance has to be struck between profitability, adequate space for maintaining inventory, and having the time for filling orders at least a few days per week.

A few years ago, I started a site along the lines of that last option. It began with a very slow trickle, but has since grown into a part-time job for one of my children (who gets paid to fill the orders daily) and some steady rewards for me.