You can create the expensive look of a hand-painted mural at a very low cost with a paint-by-number kits. Online sites (see Resources) carry a wide variety of kits that would fit the decor of any baby's nursery. The kits include easy patterns that can be traced onto the walls. Then you paint the pattern using acrylic paints. You can select patterns suitable for boys or girls or instead create a gender-neutral image.
You can jazz up a nursery in almost no time using wall stickers. Find wall stickers at craft stores or online sites such as Etsy or the Land of Nod. Even Pottery Barn carries several different styles. Peel the backing off and apply to a smooth surface. You can move them around as needed and it will not pull the paint off the walls. You can decorate with a big tree and birds, sports themes, teddy bears and so forth. Some sites such as Etsy will fashion a wall sticker from your own design.
Stenciling a mural on your wall is a longer project, but the outcome will look like a custom hand-painted mural. Purchase stencils at a craft store or online. Make sure the wall area you are stenciling is clean and smooth. Using a pencil, trace the stencil onto the wall and fill in with a flat paint. Add colors to go with your nursery theme. You can stencil a picture, phrase or even baby's initials. This will give your mural a unique and expensive look, without having to spend a lot of money.