Crab is a very delicate flavour and, so as not to swamp it, I'd just go for plain salad: lettuce, cucumber, etc., without dressing. Maybe a dash of lemon or lime, as suggested above, is OK, but no more than that.
Bear in mind that for some people crab is an allergy risk, and that for some Jews and Muslims it is also unacceptable. Probably, if many guests are expected, wise to label the sandwiches!