In last several years I have visited or organized dozens of conferences. For a long time I have been ignoring the topic of food at a conference. Two years ago I converted to vegetarianism (thanks for the trigger @lkuczera). It also influenced the quality of food I ate.
A conference is a unique experience for my body and brain. I'm sitting for the most of the day, having very little move (short walks during breaks). On the other hand the brain is working very hard, focusing, listening, understanding and remembering stuff. And what kind of fuel am I providing to it? Cookies, pizza, sandwiches, pasta and sweets. Not to forget drinking a lot of coffee, and soft drinks. In the evenings, during conference parties there's a lot of alcohol and junk food too.
All of that doesn't sound too good. But let's see what actually is important for our brains to work properly.
Food vs concentration
The food described above is mostly saturated and trans fat, monosaccharides, caffeine and alcohol. Each of them has different influence on your brain (both long and short term). Glucose (a single sugar) is a basic brain food. Brain is a main consumer of glucose in body. On the other hand fructose (also a monosaccharid) is being responsible for a large part of obesity in modern societies. I wasn't able to find a concrete answer to whether the latter is good to brain functions in the short term. It surely activates parts responsible for hunger and make you more willing to eat too much though. Fat is necessary for brain to work properly. When you are fasting for more than 3 days, your body digests fat only (your own storage of energy). Some says it is the best way to get into a long period of flow (high concentration). Pythagoras systematically starved for 40 days! Nevertheless fat available at a conference is often coming from junk food - mostly trans and saturated. Trans fat makes damage to your brain and should be generally avoided. Dehydration (common to having hangover) causes your learning and memory to suffer
A very subjective and non-representant view of mine, based on some of the events I went to, best talks I recall are often in the morning and just before lunch. This sense of correlation might be completely misleading, or caused by many other things, therefore I started experimenting with not having a lunch (but hunger was distracting), having only a small amount of food (mostly veggies, worked pretty well on maintaining focus to the end of the day) and eat food replacement (more in this at the bottom).
... duh! But to release it for a longer time, you might consider choosing Green Tea over coffee.
... for the most of the day, to sustain glucose injections. Choose those with the highest amount of glucose, e.g. banana, avocado, plum, apricot.
... as the increased consumption of caffeine cases magnesium depleting. I choose sunflower seeds, almonds and oatmeal.
Fat and Omega-3 acids
... in flax seed, avocado, oil of olive and nuts.
... especially when drinking alcohol, to keep your body and brain hydrated properly.
Since conference is often far away from my kitchen, I have a limited options for choosing food. I recently tried to choose food replacement product (a mix of nutritions, vitamins and minerals). I have to admit, that I am extremely satisfied with the effect. I was able to be fully aware on 4-5 talks a day (that's a lot in my case) and still have some energy left for networking in the evening. This became my second favorite option.