Formal:
* 今天天气真好 (jīntiān tiānqì zhēn hǎo): This literally translates to "Today's weather is really good." It's a common and polite way to express that it's a nice day.
* 今天阳光明媚 (jīntiān yángguāng míngmèi): This means "Today is sunny and bright." It's a bit more poetic and emphasizes the pleasant weather.
Informal:
* 好天气 (hǎo tiānqì): This simply means "Good weather." It's a casual and straightforward way to express a nice day.
* 今天真舒服 (jīntiān zhēn shūfú): This means "Today feels really comfortable." It emphasizes the pleasant feeling of the day.
Other options:
* 今天很美 (jīntiān hěn měi): This means "Today is beautiful" and is more poetic.
* 今天真棒 (jīntiān zhēn bàng): This means "Today is awesome" and is a more informal and enthusiastic expression.
The best way to say "nice day" in Chinese depends on your specific situation and who you are talking to.