Tree ornaments and seasonal decor are usually a safe bet if you don't want to go the wine/chocolates/houseplant route.
You can find nice ornaments for a variety of prices and styles at Michael's, JoAnn's, Target, and also KMart (although I've found more bulk ornaments here). Department stores also tend to have large, unusual and elaborate ornaments.
As for the traditional hostess gift, you could always get a nice bottle of olive oil and balsalmic vinegar (in case they don't drink), or perhaps an ethnically Jewish delicacy they might not otherwise have (ie. traditional bread, cookies, something like that. We have a large Jewish community in this city, and challah bread is often brought in for pot lucks, and it doesn't last long.)