A complete monitoring visit, run the way a real one runs: the station reference re-established, the census, the water column instruments deployed, and the photo points identified and documented.
Choosing photo points is where most people go wrong, and the error is always the same. You will want the most interesting piece of reef — a photogenic colony, a dramatic overhang. You want the most workable one: flat enough for four charts to lie near one plane, with bare substrate beside it to set them on, identified by a feature likely to still be there in ten years.
Afterwards you write the documentation. Do it before the day ends, and write down what you nearly forgot — the detail you had to stop and think about is the detail a stranger will not have.