Prep any food you can __ Buy those perishable things you couldn't buy before. Cook that food you couldn't cook before. Chill that champagne you couldn't chill before (without taking up your entire fridge with bottles). Get the decor together : Prep any decor that you can pre_prep, and get anything together that's going to need to go to the venue.
Unless it's a surprise, involve the bride in the planning process. Ask her to create a registry if she hasn't already done so. Maybe she has a particular shower theme in mind, like a lingerie shower, a kitchen item shower, or an around_the_clock shower in which guests are assigned a time of day for selecting a gift.