Rental income – A small church in a small town knew their tithes would not cover their expenses, so they bought a duplex that has generated rental income for years. A chamber of commerce rents its boardroom to non-members.
Franchise – A school ran a franchise side business so students could attend at no charge.
Gift shop/merchandise – I know of several charity organizations that have thrift stores or bakeries as one of their revenue streams. If this idea fits your situation, look into Fiscal Trade Sponsorships as these side businesses can run under your nonprofit by way of a fiscal trade sponsorship.
As always, I am hoping for your every success!