Why is 很早 after the verb in 我起床很早?
I'd take 今天我起床很早 as the shorthand of 今天我起床起得很早.
To me, 今天我起床很早 isn't my favourite sentence.  The succinct version doesn't necessarily mean &quot;more idiomatic&quot;. Some grammar pedants might even frown at 今天我起床很早 because they might take issue with the omission.
Personally, I usually say: 我今天起得很早，我今天很早就起床了 or 我今天一大早就起床了.