"The weather is nice today."
今日、天気がいいです Time words are often used without particles. This should be accepted.
Why is it は instead of の for referring to today's weather? Would 今日の天気はいいです not work?
It's basically just saying "Today, the weather is good." rather than connecting "today" and "weather".
今日の天気 would make "the weather of today" the subject, where as 今日は天気が makes "today" the subject and "weather" what is being said about "today". May seem like the same thing, but if you were to change the nouns the whole meaning changes. Example: ひとみの自転車は好きです means "I like Hitomi's bicycle", whereas ひとみは自転車が好きです means "Hitomi likes bycicles". Hope it helps.
I learned that it should be いいお天気 rather than just いい天気. Is adding the お too formal?