Formal:
* Какой прекрасный день! (Kakoy prekrasnyy den!) - This is the most literal translation and is quite formal.
* Какая прекрасная погода! (Kakaya prekrasnaya pogoda!) - This emphasizes the weather specifically.
Informal:
* Какой чудесный день! (Kakoy chudesnyy den!) - This is a bit more casual and means "what a wonderful day!"
* Какой классный день! (Kakoy klassnyy den!) - This is even more informal and means "what a cool day!"
* Прекрасный день! (Prekrasnyy den!) - This is a shorter, more casual way to say it, similar to "Beautiful day!" in English.
You can also add a personal touch by saying:
* Какой замечательный день для прогулки! (Kakoy zamechatel'nyy den dlya progulki!) - "What a wonderful day for a walk!"
* Какой солнечный день! (Kakoy solnechnyy den!) - "What a sunny day!"
The best choice depends on the specific situation and your relationship with the person you are talking to.