I am already planning on at least attempting to take them to lunch in January (the honeymoon isn't until Feb) so that I can give them the keys and all the info they need for the condo (door code, local restaurants, how to use the washer, etc. etc.)
There are a ton of maps and guide books already at the house and I fully expect the cleaning lady will stock the fridge. I went there last month and when I got there, there was a fridge full of food, soda, juice and water. When I was leaving, I left the cleaning lady a very nice tip and a short note about my friends coming, so I am sure she will treat them well.
I also left a, although somewhat cheap, bottle of champagne with a "congrats" on it for them...
OK, knowing more, you aren't being cheap by not giving them a monetary gift in addition to the vacation home.
If your parents knew the couple and were giving them their place and you were riding their coattails, then you should give them something else. Since you've arranged everything for them and your parents don't know them, the gift of lodging is from you and you shouldn't be expected to do anything more for them.
The champagne in the apartment is a nice touch. They can buy their own guide book. As a matter of fact, if someone was giving me their place in another country, I'd already have the guide book that I'd be studying.
For someone you barely know-that is an over-generous gift



