There is a bewildering array of “sports nutrition” products available today, much of it very expensive, but fortunately for Randonneurs their events generally allow time to stop at roadside shops and cafés (and even pubs !) to purchase and consume “real” food.
Over time you will discover what “works” for you – and what doesn’t, but as a starting point go for carbohydrate-rich items on the menu (cake & beans on toast are highly favoured) rather than high protein/high fat choices like the full English breakfast.
Many Randonneurs have found that flavoured milk-shake type drinks can be useful if your stomach is just not accepting anything else.
Carry with you a small quantity of confectionery (Jelly Babies are popular) or something like flapjack to replenish depleted energy reserves if it is taking longer than expected to get to the café or control. Or, and only if you know they work for you, an “energy gel” or two.
Drink is probably more important than food. Randonneurs often carry two bottles (bidons), one with plain water, one with either diluted fruit juice, or some patented energy or electrolyte drink. Make sure you keep drinking, you will lose a lot of liquid through perspiration, even on a cold day. Don’t forget to take your bidon off your bike and with you into the café or control – they’ll almost always be happy to refill it for you.
If you have a mechanical or puncture: eat something first; you'll be wanting the energy when you set off again, and it's likely to be last time your hands are clean for a while.
If you see anything on offer which you really fancy/crave, eat it, even if it seems bizarre. It probably contains something you need.
Golden Rule (whether in drinks, "Sports Nutrition" products or real food) – beware of trying anything new when you are on a long ride.