I've complied 6 things - each under $50 - that you can do to your home/apartment to make it more "adult" without the daunting task of shelling out for new furniture or buying a washer/dryer (who knew those were so expensive!!).
Painting a whole room can be expensive and time consuming, but ONE wall can take a quart of paint and cost under $30. That accent walls makes it evident that you are investing in your home without breaking the beak. And, if you are still sleeping on a metal bed-frame with no headboard, paint the wall behind your bed to give a focal wall to your sleeping arrangements.
Nothing says "cheap and boring" like the notoriously dim "boob" light that most places accumulate. BUT, did you know that a ceiling fan can be as little as $44! You can install it yourself AND it makes the space look MUCH more put together and "adult". Plus, you will save on your electric bill in the summer by not running your AC unit as much. We are slowly buying fans to replace the "boob" lights in each room of the house!
The transformation in the boy's bedroom is all due to a $30 can of paint and these $12 removable batman decals I found on Amazon! Look at those before and after pictures. Sure, the rug was a big improvement as well, but the blue accent wall I mentioned above and the decals really livened up the room for less then $50.
The mention above of taping a sheet to the window as a shade is a real life story from my husband's apartment when we met! Shades are way less expensive then you would think and add a huge "finished" look to your space. When we first moved in, we bough temporary shades from Home Depot for $5 a piece which are cheep but still look clean and serve their purpose. If you have a little more to spend, Hampton Bay makes a great quality, cordless shade for only around $13 each.
Anytime baskets are involved, your home will be more organized = more like an adult home and less like a dorm room. Piles of papers, toiletries, shoes, makeup, etc. put it in baskets. Also, turn an end table without much storage into a 2 "drawer" piece by adding a basket underneath (see picture to the right below).
Plants are not expensive but will instantly liven up a home and make your space more inviting. If you do nothing else in your new place, buy a plant and then keep it alive. Your parents will be so impressed when they come by to see how much of an adult you have become!