Cardigans and those sleep sacks are probably of the two expensive in a ridiculous way items that are incredibly useful. They're also easy to make.
Diaper bag! Unless you spend some many, a lot of diaper bags bought from the store don't last. Make something nice and sturdy.
I had these lap pad things that were super handy. They were fleece with a rubbery core. Maybe do PUL sandwiched in fleece? The squares were maybe one foot by one foot. You could use them as burp clothes, use them to change diapers, use them to mop up the inevitable milk or formula that seems to spill everywhere...lots of possibilities.
Also, you could do a whole Boppy. I've heard they're not terribly hard, and you can customize it to fit you - I'm pretty thin, and even I stretched out the Boppy I had.
(Boppies are handy, even if you're not breastfeeding. Your arms will get tired from feeding!)
A mobile might be kind of cute. Those mobiles at the store that have all the black, white, and red danglies are expensive. You could easily sew (or heck, cut out with paper if you end up strapped for time) little danglies. Some animals would be cute.
You could also do some cloth-type books. They don't really have to say anything - maybe sew in some different textures and little appliques and things. When baby gets a wee bit older and starts reaching out to touch things, s/he will like to touch and chew on the book.
Grocery cart cover - kiddos will start chewing on the grocery carts when they're old enough to sit in them. Also a pricey item!
If you plan on breastfeeding, some tops couldn't hurt. I had some cool nursing camisoles that doubled as bras that I bought from Target. With some elastic and a bit of know-how, they wouldn't be hard to make.
Here's a neat site:http://www.sleepingbaby.net/jan/Baby/index.html