They are both natural English, but have slightly different meanings. "she does" means that this is her regular habit. "for her to do" means that this is her plan for the day: she probably does it every day, but it would be possible to follow this your sentence with one that says she cannot fulfill her plan, e.g. "But washing your face isn't easy when you live in the desert."

after she gets up, the first thing she does is water the plants. GOOD After she gets up, the first thing for her to do is to water her flowers - NOT GREAT In this instance, I would say: After she gets up, the first thing she needs to do is water her flowers.