Food to bring to work and heat with a microwave
Pretty much everything.

Pasta and Rice dishes are the easiest to bring to work

Vegetable dishes can also be fun (either cooked or raw (salads)

Cooked chicken is also very easy to prepare in advance and can be eaten cold or hot; other meat.

Fish/seafood is maybe an exception, I tend to prepare and under cook a portion the night before to finish cooking the the office microwave.

If you have a fridge at the office it make things easier (and safer); and if allowed, you can keep things like condiments, sauces for a few days.