Formal:
* Доброе утро, солнышко! (Dobroe utro, solnyshko!) - This is a literal translation of "Good morning, sunshine!" and is quite formal.
* Доброе утро, свет мой! (Dobroe utro, svet moy!) - This means "Good morning, my light" and is also formal.
Informal:
* Доброе утро, солнышко! (Dobroe utro, solnyshko!) - While literally the same as the formal version, this can be used more informally with close friends or family.
* Доброе утро, любимый/любимая! (Dobroe utro, lyubimy/lyubimaya!) - This means "Good morning, my love" and is a very intimate greeting.
* Просыпайся, солнышко! (Prosypasya, solnyshko!) - This translates to "Wake up, sunshine!" and is a playful and affectionate greeting.
Other options:
* Доброе утро! Как спалось? (Dobroe utro! Kak spalos?) - This means "Good morning! How did you sleep?" and is a good alternative if you want to be a bit more casual.
* С добрым утром! (S dobrym utrom!) - This simply means "Good morning!" and is a very common greeting.
The best way to say "Good morning, sunshine" in Russian depends on the context and your relationship with the person you are talking to.