Mushrooms are actually the most well-known food that should not be reheated. Mushrooms are supposed to be eaten after preparation. If you do feel necessary to eat them the next day make sure you eat them cold. Their composition of proteins will change after sitting. They can also cause health problems including digestive issues.
Re-heating spinach is very dangerous. Spinach contains a high percentage of nitrates, but when you reheat spinach you actually turn them into nitrites. This is also carcinogenetic to the body. Spinach is better to eat right after preparation.