It’s nice to give your car a little personality, so it’s fine to leave handy or fun things in the car (eg. GPS, CDs, mobile phone holder and charger, tissues). But it’s probably best to take out anything really valuable. Personal contents are not covered under the fleet comprehensive insurance policy.
And remember to keep it clean and clutter-free. Borrowers can reasonably expect to have use of all the seats and boot space, so they won't be too pleased if they get to your car and find it full of stuff already. Repeat borrowings is what you're after!