My goal is to not be relaxed while I play but be alert but to really answer this question you will need to experiment with different things that make you feel the best while playing. I know what works for me will not work for others. Example is that I never eat before I play or during playing. Eating always seems to sag me down and make me sleepy. However I am also a person who eats only once a day and have done so for about 20 years now. I say all that because you will need to find what your body likes. Some people get charged up when they eat something (Could be fruit, could be pretzels, could be anything) but that something is what you will have to figure out.
I say the same about drinks as well. Some people love energy drinks, for others like myself it makes me feel like my heart is about to beat out my chest. All I can say unless you drink alcohol regularly I would not recommend it during games unless you are training your brain to handle it while playing (meaning if you drink while you play to get yourself used to playing while "handicapped" a little then your mind will be even sharper when it is clear).
As far as movement is concerned this is the one area I think I can be absolute on and that is you need to move. Not only during breaks but during the hour. If you play few tables then you walk around in between
hands. If you play many tables then you can at least stand and play instead of only sit and play and that will at least get your blood flowing a little bit.
Summing up, try different things as far as what you ingest into your body and see what perks you up and use those. Move around as much as you can during play and definitely on breaks.