Gnomenreigen would be a good encore, and a Chopin prelude would be nice too. I think go with a softer/slower one though, say 4, 6, 14 or 15. Maybe not 15, as I would have thought that to be included in the program, and the audience might feel a bit strange hearing it as an encore; but you never know, go with whatever you feel like.
I think end with some Gershwin of somekind, or something jazzy/bluesy so the audience have something to hum whilst they walk out.
Henrah