an activity that dovetails with the current project.
To dovetail is to fit together closely or neatly. You could just write 'an activity that fits in with the current project', but dovetailing gives the idea of a closer fit. Without more context it is difficult to give a more precise answer.

'Dovetails" seems less appropriate in a business context. An activity that is compatible with the current project, could be an option, too.