Eat fiber today, and definitely relax a lot, something that makes you laugh, like funny shows. You don’t want to work out like hell, but a nice long walk or just a few extra times going up stairs.
When you go there, think about where it is. If you can have someone drive you, relaxed, right to the door and drop you off, maybe early so you can sit in the a/c waiting room for a long time, that would be good. Bring something to read or look at on your phone that makes you happy or chuckle.
Don’t drink alcohol tonight, drink plenty of fluids. Eat both kinds of fiber tonight, starchy (oat bran, raw potato starch, potato salad...) and vegetable (have some fibrous veggies like broccoli, not just lettuce leaves.
No soda, no dessert except fruit.
Good luck!
Oh yeah: NO SODA and NO DIET ANYTHING. Tomorrow morning (if you are allowed to eat breakfast before) eat a light breakfast with protein and fruit. No cereal, no donuts and pastries. Just eggs and or meat, and plenty of water and fruit. Fresh fruit is better than juice but if you love juice you can drink a little.
Re salt: DONT GO SUDDENLY NO SALT. Your body might react weird to that. Instead, if you usually eat tons of salt, eat slightly less. Eat no packaged food that is heavy in sodium. Sprinkle sea salt lightly on what you want salted.