One of the passive income you can get is from the commission from your referrals, that would be a big help if you have a lot of them and if the site or network never fails to operate. Don't forget to help and guide your referrals or else you will lost them because they need to work for you to get the certain percentage of interest.
The hardest part is to accumulate enough referrals so that you can get enough commission out of that because mostly referrals get inactive after joining a site under you and then you can do nothing for that and first of all, it is really hard to get good and active referrals now a days.