You've gotten a lot of good advice here so I'll just chime in with a few things I've noticed over the years at our local quarry dives:
Food to share is always popular
A canopy for shade is nice too, I have a 10'x10' and it gets a lot of traffic. During cold weather I have an
E-ZUp shelter with sides (great for changing clothes!) and I put a couple of
camping tent heaters in it to take the chill off for our wetsuit divers.
A chair usually comes in handy though I'm less likely to use it if there is a picnic table available.
A friend brings a generator that powers a small microwave and refrigerator as well as a
salamander heater in the winter--I LOVE being his buddy
Carpet to stand on while changing. I used to get carpet remnants from a carpet store, they would sell you a whole box of old ones for about $10. I've also used indoor/outdoor carpet doormats (car floormats work ok too) as well as a 4'x6' indoor/outdoor rug that I bought at Dollar General for about $10. I like having a nice rock-free place to stand while getting into and out of my exposure suit. I carry a couple of extra rugs (oversize doormats) because my dive buddies kept using mine and I always had to hunt for it when I needed it
I have a bucket of fresh water for mask rinsing that sits near my gear donning spot.
I always carry a pretty comprehensive first-aid kit (tweezers are a must!) and a fairly large save-a-dive kit.
Ber :lilbunny: