Something green for vegetables....asparagus or Haricot Vert.
To offset the saltiness of the sea.....I would also suggest a salad eaten along with, or after the crab with a slightly sweet salad consisting of any of the following or all:
mix greens or mesculin with radicchio and endive
goat cheese or Gorgonzola
candied pecans or walnuts
apples or pears
orange or tangerine sections
heirloom, cherry or grape tomatoes
I also like (Analisas mom)'s idea of corn.....maybe a mango corn relish....sweet on its own...or spicy with the addition of some fresh Serrano or Jalapeno peppers.... or just chili and/or cayenne pepper(powders).
I do snow crab every year for my wife and daughters and this year is no different. Although for V-day it ends up being all about the crab. I have 6+ pounds for 4 of us with sliced lemon and melted butter. The one thing I do include always is a greek salad. Just always seems like a good combo for us. During the non-V-day dinners of crab, I just include a nice bread to go along with it. My girls are carb fiends so it stretches the crab a lot further.