Great question! if the mother being celebrated has a favorite dish or favorite restaurant, those are always good ideas. If she loves going out, surprise her with reservations for a place she loves.
Another idea that came to mind when I read your question is outdoors grilling. Men love to do that (at least the commercials all say that's true!). May is the month when the weather finally lets up a bit, so grilling a meal might be fun and creative.