Hosting Shower in the Slope
we did a bridal shower with about fifteen people at patois on smith street last year. it was wonderful. they'll probably want it to be mid-afternoon, between the regular brunch hours and dinnertime; i think it was 3 o'clock or so. we sat in the front room with the big windows open and it was perfect. we also considered chestnut but i had a more difficult time getting a hold of them and so am not sure what they would offer you.
If you're aiming for nice weather and out of doors, the back patio at Le Petit Cafe would be very nice. This place is best for brunch anyway. The garden out there is large, since you didn't specify the number in your party, so they will probably give you a section that fits. It's the prettiest garden in the whole neighborhood IMHO.
It's on lower Court in Carroll Gardens. Parking is very easy in that area. Near the F train.