if you want to express that you are fine today, you can say"今天感觉不错"，if you are in hospital, then you can say"今天我身体很好" to the one who visit you. it means you feel healthy today

There are many ways to say this. Usually we rather just upgrade fine if you are very fine. Upgraded words would be- in order of dong better each time - OK, fine, better, much better, doing well, pretty good, great, super, really awesome. These are all typical ways of saying how you are doing. Notice very fine is not in the list. Very fine = much better if you were sick before or "doing well" if you were not sick. Very fine is a slang term for beautiful - just so you know. Even if you are "very fine", in this way, you still might not say this about yourself.

I'm healthy today.

it is right.but usually we say ：今天我很好. no "身体".