You are probably wondering what changes add value to a 20th century home(one of the criteria for listing on the National Historic Register is that a property be at least 50 years old). So put yourself in the place of a buyer, and think about what you need in a home to make it functional for your family. Unlike the very early home buyer, you expect 1 ½ baths, and a modern kitchen with an eat-in area. And a pre-1970 home should have oak or pine floors. If they have been covered by carpets, this is a good time to uncover them and have them professionally refinished with a low gloss (high gloss finishes shout "look at me"). I can't count the number of times buyers have tried to pry up a corner of carpet to see what kind of floor is underneath. And let go of the wall to wall sculptured or shag that is 30 years old.
Bathrooms should have either pristine original fixtures and tile, or some of the contemporary versions of subway wall tile, hexagonal floor tiles, and pedestal sinks (all enjoying revivals presently). Kitchens seem to inspire more creativity, and most people seem to lean toward stainless steel appliances and granite countertops, and room for the family to eat together.