We (wife, kids, and I) have a variety of bottles, including some Polar and Camelbak insulated bottles, which are a little big, but sure keep things cool and hold a lot. I have a smaller non-insulated Specialized “Purist” or something that fits like a glove in my road bike cage, and I really like those.
I’d also say get some good padded gloves. Have had Gore, Pearl, Giro... they’re all good.
If someone has an under-seat bag that’s helpful too.
And make sure you have a good, solid pump with a gauge on it that can deal with both Presta and Schrader valves.
Tire levers and spare tubes also = a good idea!