'What did I do this weekend?' is translated, word for word, into 내가 이번 주말에 무엇을(뭐/뭘) 했지? You know, you can be forgetful sometimes, and this is the right one for that situation. :D But, it can also be translated into 내가 이번 주말에 뭐(뭘/무엇을) 했게? 추측하다 is for the word Guess. When 'guess' is used as imperative, 추측하다 is changed into 추측해 봐. So the sentence 'Guess what I did this weekend.' means 내가 이번 주말에 무엇을 했는지 추측해 봐. This Korean sentence has a little bit of formal vibes. Between close friends, 내가 이번 주말에 뭐 했게? is used. Guess what I did? 내가 무엇을 했는지 추측해 봐. > 내가 뭐(뭘) 했게? Guess what I ate? 내가 무엇을 먹었는지 추측해 봐. > 내가 뭐(뭘) 먹었게?

these two sentences "what did i do this weekend" and "guess what i did this weekend" are totally different, I think. the first will be "what did i do this weekend, can you guess?" to be the same meaning as "guess what i did this weekend?". both will be "내가 이번 주말에 뭘 했게?" in Korean. here is a secret.:) if you put "게" at the end of sentence with a question mark together, it will mean " to ask somebody to guess what i did or what i meant to do something". this can be one way to be translated as "guess" in English. for example, 내가 뭘 했게?(한번 맞춰봐= can you guess/imagine?) guess what i did? 내가 지금 뭐하고 있게? guess what i am doing now? 뭐게? guess what?

translate liberally : I have interesting story on weekend. Are you ready to heard ?? Have a nice weekend : )