Formal:
* À demain, mon cher/ma chère. (This is the most neutral and polite option.)
* À demain, mon amour. (This is a bit more affectionate but still polite.)
Informal and affectionate:
* À demain, mon cœur/ma chérie. (This is more intimate and sweet.)
* À demain, mon/ma trésor. (This means "my treasure" and is very affectionate.)
* À demain, mon/ma chou/chouchou. (This means "my darling" and is very familiar.)
You could also say:
* J'ai hâte de te revoir demain. (I look forward to seeing you again tomorrow.)
Remember to choose the option that best suits your relationship with the person you are talking to.