For a fun and creative way of decorating on a budget, think construction paper. Yes, there are a lot of things you can do, but putting a swarm of bats in your house, and a pretty cool way of getting that slightly spooky effect.
I found you can get an entire package of construction paper in just a single color at AC Moore, or other craft stores like that.
Next, go online and find yourself a bat template. There are a lot of free sites out there you can find. Just find the one you like and print it out.
Next, use that template to cut out a main template on something with more substance like poster board or even a box.
Once you have your template, take a sheet of construction paper and fold it on half. You can trace your bat onto one side, then cut 2 out, at the same time.
(My girls just couldn't resist decorating my template)
Once you have them cut out, fold the wings both inward, so they pop off the surface a little bit.
Lastly, get some double stick take, and tape them to the walls in a flying pattern. If you get a large amount, you can make it look like a swarm of bats flying through your house.