Mr grandfather always insisted on 36 inch wide interior doors. It makes it really nice to move things in and out of rooms, and so much easier if you ever have someone in a wheelchair in the house, which he had to use his last few years. It's a feature that is very difficult to add later on.
My dad put outlets under the eves for Christmas lights when my parents built their house. A small thing, but it makes it so easy not to use extension cords.
I'm sure the rest of the guys on here have alot of great tips like this. Many of them have built/had built their homes.