Actually, Rachmaninoff 16-5, 23-4, 23-6, and 23-10 are all perfect pieces for the occasion; I don't know how much time you have, but Op. 23 No. 10 is only two pages long, unobtrusive, and rather beautiful.
I actually have a binder of quiet music that I can take with me when a pianist is needed for some reception or another. These pieces require very little preparation or are entirely sightreadable for me. I don't know what level you're at, but I hope this helps:
RavelPrelude
Pavane de la Belle au bois dormant (from Ma Mere l'Oye)
DebussyClair de Lune (from Suiter Bergamesque)
The Snow is Dancing (from Children's Corner)
Et La Lune Descend Sur Le Temple Qui Fut (from Images Book II; this will probably require some work beforehand)
Des Pas Sur La Neige (from Preludes, Book I)
La Fille Aux Cheveux De Lin (from Preludes, Book I)
Eric Satie3 Gymnopedies
ScriabinLOTS of great pieces. Buy the Dover book of preludes and etudes; you'll find between 15 and 20 pieces that are well-suited to your needs. And if you have a little more time, there are a few etudes you could learn which would be appropriate for the situation.
ChopinNocturne Op. 9 No. 1
Nocturne Op. 9 No. 2
Nocturne Op. 37 No. 1
Nocturne Op. 55 No. 1
SchubertAny of the Impromtus are great, but would probably require a significant amount of practice beforehand.
LisztSix Consolations
This list is by no means exhaustive - for example, I've left out Mendelssohn, who wrote many great pieces suitible for this sort of occasion. I'm more familiar with the works listed above, as I've played or sightread all of them before.
If you're comfortable with it, also try improvising! I tend to improvise about as much as I sightread at these occasions.
What's most important, perhaps moreso than the repertoire you play, is keeping the atmosphere serene. Resteraunt-goers aren't expecting or even desiring a concert (unless that's what's being advertised). It's possible to play loud and fast music in an unobtrusive manner, just as it's possible to play quiet, slow music in a jarring, attention-grabbing manner.
Sorry if I've bored you with things you already know; it wasn't clear from your post at what level your playing is or how much experience you have with playing at social functions.
Good luck!

- M