Built-in wardrobes are good inexpensive renovations that get you lots of additional storage space. They're pretty simple to build, and make a good project for someone wanting to develop their DIY skills. You'll get experience in how to frame a wall, how to put up drywall, how to plaster, and how to install a set of sliding doors. The whole project costs about $150, assuming you've got the required tools already. If you don't, what better excuse?
The built-in wardrobe described here is designed for shoes (a wardshoe?), so it's a bit shallower than the average wardrobe, and it's filled with shelves rather than a clothes rail. However, the construction is pretty much the same as other built-in wardrobes I've made - this is just the only one I thought to document. It took an afternoon to build, then plastering took a week as did painting.