Build From a Neutral Base
If you kept the flooring, trim and built-ins neutral in the nursery, adding a couple of bright accent colors like bright blue or lime to update a child's room will be easy to do without a major overhaul. You may also be able to give some of your nursery accessories, like wicker furniture, trunks or baskets, a new lease on life with the judicious application of a little spray paint.
Keeping the room neutral and working some child friendly design magic on the furnishings and accessories may be your only option if you're living in a rental where you don't have the freedom to paint the walls or change the carpeting. If that's the case, you can always update the space for your child with wall posters, vinyl decals, area rugs, bedding and pillows.