Original: In my uni days, I played in a rock band and painted!
Recommended (neutral/formal): During my university days, I played in a rock band and painted.
- Formal: When I was at university, I played in a rock band and painted.
- Neutral: In my university days, I played in a rock band and painted.
- Casual: Back in uni, I played in a rock band and painted.
Notes:
- "uni" is informal. Use "university" in formal writing.
- The comma after the introductory phrase is optional; both with and without the comma are grammatically acceptable.
- The verbs "played" and "painted" are parallel and correct. If you want to be clearer, specify what you painted (for example, "painted portraits" or "painted as a hobby").
- An exclamation mark is fine for emphasis in informal contexts; use a period for a more neutral or formal tone.
Plenty of your original sentence is fine; the main change is replacing the informal "uni" with "university" for formal contexts and choosing punctuation based on tone.