Compile a list of very low hanging fruit - tasks which might take a few minutes each. For example things like code linting, formatting or generating docs. 20m

Compile a list of very low hanging fruit - tasks which might take a few minutes each. For example things like code linting, formatting or generating docs. 20mins is plenty of time to execute "housekeeping" tasks related to building software. Alternatively do something physical - pull-ups, squats, push-ups, etc.