My side project (https://www.teamcalapp.com) returns around $2500/month and growing. I think the main improvement to be done is doing more marketing, which is unfortunately not so passive.
Im also investing in apartment complexes. Right now i have 4 passive investments. Since I’m only a passive LLC member that income is truly passive. Those investment deliver nice regular cashflow (8-10%) and appreciation once sold. Right now I’m in the process of buying my own property together with friends (50-80units). That will then be a bit more work but still partially passive once all is setup with the property management company.