How about doing a St st blanket with a garter st border? Figure out which yarn you'd like to use. Make a swatch. Figure out how many stitches per 1 inch, then how wide and long you'd like your afghan (in inches) and you can figure out how many to cast on from there. Just knit the first 4 rows (garter st), then knit the first 4 stitches at the beginning and end of each row (place stitch markers to help you to remember). All you have to do is to knit in St st until your afghan is as long as you'd like it. Then, knit the last 4 rows in garter st.
Knitting/Crochet/Crafting/and Related Product Review Blog: KnitPurlGurl
"Yarn stashes insulate the house."
"Just one more skein can't hurt."
- from Knitticisms