Here are some things I've found useful in the past:
cereal boxes ---> postcards
magazine pages ---> envelopes, shredded up to make confetti to snuggle your gifts into
cassette tape cases ---> business card holder (decorate it too )
old kid's books that you won't read anymore ---> use it as a scrapbook, leave the cover as it is or you can cover it up. The great thing about using books is you won't have to worry about flimsy pages if you choose wisely, and they don't cost anything if you own them already
paper bags--->tags, wrapping paper, book cover etc. so many different things you can do with paper bags!
eraser---> carve a design and use it as a stamp
sweaters that you don't like---> turn the sleeves into leg/arm warmers, unravel the yarn if you can knit or crochet and recycle into a new project, or you could make mittens, scarves, and hats (etc.) by making a pattern and simply cutting it out of the sweater and sewing it up using the zigzag stitch
Chinese takeout boxes---> if they're new, they make excellent gift boxes
doilies---> i can't get enough of doilies as they are so versatile! For example, you could sew on a doilie to the front of a bag for maximum sophistication, or you could place some on a t-shirt and then take some spray paint and use them as stencils... you can find them at most craft stores for around a dollar each or you could go to the flea market and get them for much less!
Hope there was something useful here