Just to clarify my earlier comment. If it is the start of the day and you are thinking about all the things you will have to do then "What a busy day I have!" is correct, but if you are at the end of the day and looking back at it, then it would be "What a busy day I've had@

What a busy day I've (I have) had.

I agree with Bramwell. If you are looking at your schedule at the beginning of the day and noting how much you have to do, you can say 'What a busy day I have.' At the end of the day, you'd say 'What a busy day I've had'. You could also say '..I had' at the end of the day, or at a later point.

What a busy day I had !

What a busy day I have - say this at the beginning of the day when you are looking at your schedule What a busy day I am having - say this during the day to reflect on the business of your day. What a busy day I've had - say this at the end of the day What a busy day I had - say this at the end of the day OR say it the next day or sometime in the future when looking back on today.