Bleeeeeeah. I always have a stock of crime fiction scenarios that I take in with me. So, no idea what you're going to be doing. The easiest thing, in my humble opinion, is to stick to those conventions that you mentioned if that's what they asked for. And do something simple, so you can let your creativity and fresh approach to the genre shine!
Excuse me. I've eaten way too much cream today.
Ok. From me to you (not that you care):
1. Have some basic plot. Doesn't matter how stupid it is - you're not going to have to write the whole story. But get it in your head who did it, why, and the nature of your detective etc.
2. Take a new perspective. You don't have to be the detective. Of course being the cat is a bit "eh".
3. Don't do the crime. Far too hard to do effectively. If you want, you can do the detective taking on the case, or show the effect of the crime on the person's family (if you're working with a murder), or whatever.
4. Conventions, conventions, conventions. They can be overdone, so watch it. Don't slap the markers in the face with them.
5. I can't think. I'm tired and lazy.
Someone else can list the rest.