Maybe try a blanket that starts in the center and work outward, then you can knit until you run out of yarn. Figure how much yarn it takes to go around the whole way, then you'll be able to save enugh to bind off. Or better yet, knit until you run out then use a contrasting color, even if it's a different yarn company, to bind off.
Here's a pinwheel blanket: http://www.knitlist.com/00gift/round-baby-blanket.htm
Or how about one like the grandma's favorite dishcloth pattern - you start in one corner, increasing on each side until you run out of your first ball of yarn, then start the decreases. Here's one (leave off the hood): http://www.lionbrand.com/patterns/70090A.html?noImages=