Based on the level of respect or formality, you can say "What did you do today?" in Korean in 3 ways:

• Informal: oneul mwo haetseo? (오늘 뭐 했어?)
• Formal: oneul mwo haetseoyo? (오늘 뭐 했어요?), oneul mueot-eul haetseupnigga? (오늘 무었을 했습니까?)
• Very Formal: oneul mueot-eul haetsieotseupnigga? (오늘 무었을 하셨습니까?)

- what (objective): mueot-eul (무얼), mwol (뭘), mwo (뭐)
- you (subjective): neo-neun (너는), dangsin-eun (당신은), dansin-ggeseo-neun (당신께서는), ne-ga (네가), ni-ga (니가)
- to do: hada 하다)
- today: oneul (오늘)

Please refer to How to pronounce Korean Alphabet for the English pronunciation in this answer.