If you want to play relaxing pieces for them that they've heard, I see no reason why La fille aux cheveux de Lin and the Op. 9 No. 2 nocturne won't work. Both are pretty fast to learn as well.Besides that, I guess anything else from Children's Corner or the 9/1, 15/3, 32/1, 32/2, 55/1 nocturnes are pretty easy. If you want a faster, brief piece that sounds much harder than it is, the F minor, E-flat minor, C-sharp minor, and C Major preludes from Op. 28 fit.
Yes, Chopin and Debussy are good sources for background music, though I suggest you avoid the minor key pieces of Chopin - they're rather dark for a social gathering. You also might explore Faure and Ravel. Here's a few more:- Debussy Pour le Piano - any or all of the 3 mvmts- Debussy Arabesque 1 (not sure why you didn't mention it)- Debussy Estampes, Preludes- Ravel Sonatine- Ravel Pavane pour une infante défunte
Topic: Piano Pieces for around Upper Intermediate - Lower Advanced Level 