Messy French twists and casual ballerina buns are examples of semi-formal updos that are easy to do. Styles that are only half up or that feature braids are also easy while being fairly casual.
While a sleek French twist is a classic formal updo, leaving parts messy makes the style more casual. To create a messy French twist, add texture to the hair with a curling iron or styling products. Tease the crown for volume. Pull the hair back into a ponytail, and work it into a twist. Loosen the front, removing some strands from the twist, and secure with bobby pins.
For a half-up version, pull only half the hair up and back into a ponytail. Work the hair into a twist, and secure at the crown, leaving the bottom hair hanging.
For a casual ballerina bun, secure the hair into a high ponytail. Twist the ponytail around the base, and secure with bobby pins. As an alternative, pull the hair into a high ponytail, but secure only part way with a hair elastic. Wrap the elastic around the base of the tail one last time, but only pull the hair partially through. Secure this top knot with pins.
For another version of the bun, secure the hair in a middle ponytail. Braid the tail, and wrap it around the base, using pins to keep secure.