You could make a pedalled lawn-mower as long as you had low gearing. After all, there are push-along lawnmowers that are entirely human powered. However, it would have to be quite small and/or slow and so would take a while to get the job done. One option would be to have a pedal-powered 'prime mover' towing a mower/other implement, with the trailer machinery being pedalled and controlled by a second person on the trailer.