Hi Ioannis,
There's a few things I would suggest.
- Investing on a better mattress and pillows for sure. I think any other spot in the flat is better that the current mattress.
- Making the kitchen a bit more tidy. It's good that there are plenty of things to use but it is a bit soulless. That way it can compensate for the really small bathroom which now make sense that you did not have an image on the listing.
- Keep spare bed sheets and covering in the wardrobe just in case
- Make it clear on the listing that the apartment is a basement
- See what you can do with the smell