You could probably chug water but don't go crazy dieting. (I never believed my husband when he'd tell me "water weight" but I notice if I eat out (and keep track of my sodium too) that the scale would read higher the next day!) Once I had 5,000 grams of sodium in one day due to eating at a casino buffet! I think I jumped 1.5 pounds in ONE day, but then dropped it the next day. Since a KG is roughly TWO pounds and you ate out a lot 2 days, that makes sense.
Have fun at the gym - I know you said you go early :)