Lunch in Park Slope
It very much depends if you are coming on a weekend or during the week. On the weekend there are plenty of good brunch options- mostly on 5th Avenue, as txfireant said. During the week there are less choices, as the best restaurants are closed, but there are plenty of decent if not spectacular eats. Tell us when are you visiting and we can give better input.
A belated thanks for your suggestions. We ended up going to Rose Water, on Union Street. If I remember the dish correctly, I had a special of duck confit hash browns covered with an over-easy egg and accompanied by a salad with cranberries. The dish also had marinated fennel on top, which sounded intriguing but tasted odd. All in all just what we wanted -- good food and a relaxed, neighborhood feel.
Try the 5th Avenue restaurants over the 7th Ave ones. Go to the new A.O.C Bistro on 5th between Garfield and 1st. Like A.O.C, Moutarde is french and one block away between Garfield and Carroll. Go to Moutarde for brunch (banana pancakes, salade nicoise, omelette with feta and spinach), as dinner is very mediocre. Next door to A.O.C Bistro is La Villa, which is good for a hearty meal. I'd go for the entrees, not pizza, at La Villa.
If you end up on 7th Avenue shopping, try the Italian Sotto Voce. For a quick bite, there's Roma Pizza and its Grandma's pizza. Chiles and Chocolate has delicious enchiladas for a decent price.