I hate those questions. It's the kind of thing you have the perfect answer for 5-10 days after you needed it.
A lesson I learned from resume writing....
You think of things you've done (as resumes should highlight accomplishments and not just mundane job duties...anyone who is a secretary can probably do dictation, but if you managed 150 files single-handed, that latter bit is more intriguing to an interviewer).
It doesn't have to be something you got paid for. Something you do for yourself or volunteer work counts. If you've done something most people don't do that might be interesting, write it down and later look back and see if any of it might apply to a work situation.
In time, you have a "mental list" of unique things you've done that might impress an interviewer. I can claim to have managed upwards to $3,000,000 in court-held funds because at one point, the total we had in all accounts was just over $3,000,000. Nobody needs to know that was for a short time and later it was rarely over $500,000. Sounds more impressive than saying I noted quarterly interest earned on 100+ files every three months as part of my job.