(Contrary to what this image may suggest, I actually recommend staying clothed when you visit people during the holidays.)
I’m a big believer in coming gift-in-hand to any and all holiday get-togethers – even if all you tote along is a bottle of wine (which no one will ever be sad to see), it lets your hosts know that you’re appreciative of the time and the effort they’ve put into the event, whether it’s a cocktail party or a week-long stay.
My general rule: tailor the size of the gift to the duration of your presence in their home. For a regular old dinner or cocktail party, a bottle of wine or a bottle of decent liquor is sufficient (assuming your hosts drink or are serving alcohol) – flowers force your hostess to stop whatever she’s doing and trim stems and find a vase.