A big +1 on the sun protection and water.

On road trips everyone else would bring the beer. I would supply a part of the ice and stock water. By midday people would be trading beer, and pretty much anything else, for water. Slamming beer in the Florida sun is good for a time but you could just about die of thirst doing it.

Wide brimmed hats, light long-sleeved shirts, sunscreen, zinc oxide, sunglasses. I've found it good to toss in a couple of $10 discount store fleece tops and a fleece blanket big enough for three. After a day of partying sundown can feel mighty cold.

The blanket also works as cover for relief into a bottle. Get a Freshete or Lady-J for distaff use. Undercover relief into a bottle can prevent unpleasantness associated with public exposure/urination charges and mile-long port-a-potty lines. Carrying an adapter for females use into a bottle gets you big points with the ladies. As we all know, if the ladies are happy everyone is happy. Might want to keep a supply of condoms on hand. Not that one necessarily leads to the other. But things happen and safe is better than not safe.

Include a couple of pairs of socks. Tops of feet get sunburned, feet get cold, people seem to lose their shoes and the half mile walk back to the car is easier if they have something to protect their feet from the inevitable broken glass.

Garbage bag - Emergency rain wear, a dry place to sit after a rain, a place to stash your trash, a good place for that guy in front of you to puke (into a bag is better than your lap), a place to stow the clothes that come off as the day warms, a way to hide the body.

Ian is right about keeping the gear cheap and expendable. Concerts foster a strong community property atmosphere. Gear in a duffel with someone sitting on top is pretty good protection but no guarantees.

Dehydration, sunburn, small scrapes and lacerations, and sprains of the ankles and wrists tend to be the most common injuries.

The first-aid kit would be:
Water - a few half-liter bottles as reserve.

Aloe - Soothing for sunburn.

Zinc oxide - sooths sunburn and prevents more sun damage.

Alcohol swabs - Bandaids and tape won't stick unless you clean off the grease.

Bandaids - Fabric, box of 3/4" ones and a few fingertip and knuckle.

3by3 gauze pads - I like to go with one large size and fold or cut down for small jobs. A couple boxes if your using them for everything. Others might carry fewer large ones and use 2by2s to wipe and clean.

3" elastic bandages - support wrists and ankles but also hold gauze for larger wounds. Get ones with or carry safety pins to hold them closed.

Athletic tape - At least a couple of rolls. Cheaper, stronger, better sticking and more adaptable than adhesive tape sold for first-aid. Easy to rip for thinner pieces. Tape and a bit of gauze and you have a bandage. Duct tape is also good. Handy to tie hands and/or feet of freak-out cases.

Triangular bandages - Bandage and binding for wounds, material for tourniquet if it comes to that, cover for sunburned feet, blindfold, gag for biters and screamers, lay one out for a clean place to work off of. Fairly efficient restraints.

Betadine swabs - quick and dirty wound cleaning. You can do the same job with 2by2s and a bottle of betadine but it can be messier. Less expensive on a per use basis but messier.