Well, unfortunately, it's pretty much wide open. It depends on the time of day the mother's preferences, who's invited and your social circle. I would definitely stick to finger foods, and it's a whole lot easier if you avoid mealtime. Drinks-I had/hosted showers where there was alcohol served (beer, wine, sparkling wine and frozen drinks), but just made sure there were fun alternatives for the mom/non-drinkers (juice drinks in martini glasses, virgin frozen drinks, gingerale type punch). Others (hosted by the church crowd or where my Mom was there) stuck to pretty much water & tea and maybe some sort of juice or soda based punch if you like.
A meat/cheese/fruit/veggie trays are easy and great. Don't forget the crackers. Finger sandwhiches are another option if you want to go for a "meal" or heavy appetizers. Other finger foods that go over well are nuts, and anything from the pilsbury crescent roll recipe website-lots of good stuff there. We picked up hot wings for one of my friend's showers, because she loved them. A friend of ours steamed or grilled shrimp for one. So anything goes, really. Meatballs kept warm in the crockpot are good-I have found a couple of good recipes for those over the years. Some sort of little cracker "sandwhich" like are always pictured on the Ritz cracker box would be fun too, but kind of labor intensive. Definitely throw in a bowl of pastel m&m's for fun and for the sweet tooth. With a March shower, you should be able to find little pastel candies that come out prior to easter to set around.
Then of course, there is the obligatory cake.
Have fun. Fortunately, you still have time to plan.