may be a little late for this, but try and drop any expectations you have for the race. I know you probably have a goal time based on what you've been doing in training, but try and let that go. I did that for my first Half earlier this year and had a GREAT experience. Obviously I had a goal time in mind, but I didn't let that thought creap into my head when things slightly went down hill and I had a great race despite a sub-par run. oh yea, hydrate VERY well on the bike. The run is longer than you think (personal experience).

My first is also next weekend. I am trying to calm down and focus on:

1. Being confident in my training plan.

2. Realizing that I can't do anything at this point to improve upon my training, but I can screw it up by working out to hard and failing to taper.

3. Taking it one discipline at a time and not thinking about the total mileage.

This being said I am as nervous as I have ever been.

Good luck on Sunday.

Start getting a good nights sleep now! As it gets closer and closer, you may stop sleeping a soundly, but you can start making sure you're well rested in advance, instead of one day before. Taper this week, but try to get some short workouts in all 3 sports in, doing very short tempo work to help your body remember how to do go.
Don't get too caught up in the day before registration and stuff, walk around a little, but try to be easy on your feet and stay out of the sun.
Save your hottubbing for after. The one marathon I've hot tubbed before, my legs were tired and slow for.

Most importantly go out there to have fun and finish - this will always be a PR for you and if you go too fast you may not finish (or you may set a time that is really hard to ever beat!). You've done your training and just take confidence in that you're ready to do it!
Lots of luck!