Why not sizing culverts or small flow treatment plants? Here in PA, the culvert and wwtp plans need to be stamped by an engineer in order to get a permit. I know of quite a few engineers who freelance on the side doing small flow wastewater stuff.
Heck, there's an SEO here in one county, who does the designs for small flow plants, and has an agreement with an engineer to stamp his drawings. The engineer get $1000 per stamp... pretty shady, though, if you ask me.
I would do it myself (not stamp SEO drawings, but freelance), but it's conflict of interest right now. Maybe in the next year if I get a new job outside of government.
Brian