Corrections in red.  Stylistic suggestions in blue.

Holiday

Currently, I'm visiting my daughter in the USA. I have been here since July and will be staying until the end of September.  What I love the most is that the sun is always shining here in California. The weather in Germany won't be sunny anymore when I return home from my trip. So I'm enjoying the California weather to the maximum [alternative: ... weather as much as I can]. And by the way, I can practice my English during my stay.

If you say, "I'm enjoying it the best I can," it suggests a nuance that you are having difficulty enjoying yourself, but you are trying very hard to enjoy yourself. As a Californian, I think (and hope) that's not what you mean!