DIY signature cocktails. Obviously lemonade wouldn't make sense for Prohibition, but it's a cute setup. You could always do different punches. Plus, mixed drinks can be cheaper because you can use really low-end alcohol and nobody can tell the difference.
Water Drink Station: Give guests a quick cool down before the ceremony with a water station. Fill stylish water dispensers with fanciful flavors such as lemon and cucumber, lime and mint, and strawberry.