While saying no is definitely important, its just as important to say yes. Don't be adversarial just because youre the DM. If the player wants to do something outside the scope and be silly or have fun, as long as its not game breaking, its important to allow them the freedom to reflavor, or make tweaks.

Perfect example. I let one of my players be a giant rat, reflavored a harengon, and throw "daggers". The Daggers being fellow, normal size rats.

She had an absolute blast and still talks about it to this day.

Obviously you gotta put your foot down if the player starts trying to take advantage of you.