Freelancing, but they should approach you first. Don't use platforms.

Alternatively, non-billion dollar projects are relatively untapped. There's a rich seam of million dollar ideas that VCs won't fund but are too deep to do in a weekend and too niche for corporations and agencies to do in their spare time. People have made good money off recipe apps and news apps, or some form of API that does things like live movement detection. You'll probably have to solve consumer problems though.