To be as fresh as a daisy means to feel well rested, alert and full of energy.
It can also be used to describe someone who looks bright, attractive and full of energy.
Examples of use:
1. I had a lovely night's sleep last night and I feel fresh as fresh as a daisy this morning.
2. She arrived in New York after a long tiring flight, but she looked as fresh as a daisy.
Image by Dennis Wong