Its an 800, so IMO diet could matter less.
There is some science to light full-body lifts (cleans, etc) 4-6 hours before a competition helping explosiveness in jumpers and sprinters, but i'm not sure if it translates to running. Regardless, I'd recommend an easy warm-up, along with some 50 meter sprints at 90% a few hours out.
Caffeine is always useful for the mental clarity it affords. I'd normally drink a bit of caffeinated beverage right before starting the warmup.
Speaking of which, some runners feel best leaving for their warm-up 45-60 minutes out from the race. I followed a 60-minute rule, but in looking back at my 800 and mile prs, both were accomplished on days where the team bus arrived late and I had 30 minutes max to get to the starting line. Maybe there's something to that? Whether that's panicking and therefore not overthinking things, or coming in a bit tight, I'm not sure.
I'd recommend looking into the various energy systems and how they are used in the 800m. For example, creatine phosphate will be used around 20-25 seconds from the start of the race, depending on the athlete of course. So you're basically getting a free 150-180 meters. This allows you to get out hard and coast the next 220.