BR is only at preschool for a few hours in the morning so there is plenty of time left in the day to entertain himself while his sisters are still away. For the first few days of the school year, he wasn't used to being alone and had never really learned to entertain himself. Translation: he wandered aimlessly around the house and followed me from room to room.
That was pretty fun and all, but we were both much happier when he took to building "trains" of toys or shoes or books (or anything at all). This keeps him busy for a long, long time and the results are something he can be proud of.