We clean out everything, including the fridge, clean and sanitize everything. We never leave anything left over from the previous guest.
After everything is cleaned, we place a standard set of newly purchased, sealed items, including milk, water (still and fuzzy), butter, jams, a bottle of local wine, a few beers, some cola, orange juice, and a loaf of bread in the fridge, with a note indicating it's all fresh and complementary.
Also leave out a few unrefrigerated snacky things like biscuits and crisps, tea, and something sweet to go with the supplied (capsule) coffee. Sealed. New. Always.
Guests consistently seem to be impressed. For the 20€ or so, it's a good investment.