Definitely a race day checklist. They will love you forever if you print out something they can take along with them.

I agree with the others, the swim is the scariest part for the first (or even second or third) timer. I think the best thing I was told was to start at the back or the side of the group and to swim my own speed. Don't worry about anyone else. There will be people faster and slower. Don't let them psych you out. Also let them know about all of the safety people that are around to help them if they get in trouble. Try to make them as comfortable as possible with the thought of the swim.

2+ on the portapotty laps. I go, then get back in line, go again, get back in line ...

Lastly, let them know that no matter how long or short the race, going the distance is a HUGE accomplishment.