Formal:
* J'espère que vous passerez une excellente journée. (This is the most literal and formal translation.)
Informal:
* J'espère que tu vas avoir une super journée! (This is more casual and uses the informal "tu".)
* Passe une bonne journée! (This literally translates to "Have a good day!" but it implies you hope they have a wonderful one.)
* Bonne journée! (This is a very common and concise way to wish someone a good day.)
You can also add a little extra to your greeting, like:
* J'espère que tu vas bien aujourd'hui. (I hope you are well today.)
* J'espère que tu profites bien de ta journée. (I hope you are enjoying your day.)
Ultimately, the best way to say it depends on your relationship with the person you are talking to and the situation.