You should take supplements on non-workout days, and the ones you take must form part of an overall targeted nutritional and supplement plan.
You see, supplements are nutrients, and the body can always make use of nutrients if it is under training stress. Recovery and rebuilding occur when the body is fueled with the right macro and micronutrients, including protein, carbohydrates, fat and vitamins/minerals.
So having a daily (and planned) approach to your nutrition is key to optimising your body for continuous adaptation and results.
Ensuring your body has the right amount of protein (and thus amino acids) is key to ensuring you recover and adapt from the training.
So, whether you use food (number 1 source) or supplements (protein supplements/protein powders), the fact is that you will need protein.
So you should drink your protein shakes on workout and non-workout days; if the protein requirements are needed.
Creatine needs a loading dose, following by a replenishment dose. The latest research suggests that having a steady flow of creatine where the majority is consumed pre and post-workout is recommended. The non-workout days you can consume between 1 and 3 grams.
Yes, you should take creatine on rest days but only a smaller amount.
You should consume amino acids on rest days, even if that’s from foods or supplements. Amino acids are the building block of muscle recovery and muscle building, so fueling the body with these macro’s is key to ensuring the body recovers.