I don't understand what WHAT ARE YOU UP TO and WHAT DID YOU COME TO have to do with this question.

Dear Uly, the Russian Что ты решил? may be an inquiry for the future plans or intentions, while What did you decide? for me sounds just like What did you choose?

I agree 100%. We say the same thing "so, what did you decide (on)?" – Especially when you want a yes or no answer. But "what are you up to?" means "Что ты делаешь (прямо сейчас)?" A better question in this context would be "(So) what are you going to do?"

Thank you for making it clear!

Anytime, Dmitry!