I took a skein of yarn that matched my bedroom and simply made a giant rectangular "doily" from it. I just made it in double crochet for as long as the yarn lasted. It looks great on the top of my high-boy dresser and adds a bit of color to my white-walled room. There's tons of doily patterns that could be made up in just about any yarn if you look around a bit.
I made a huge granny square blanket (just kept making rows and rows and switching colors, not individual squares) to match my room. I had a lot of purple Vanna's Choice (my very favorite yarn ever). I'm finishing it tonight and it's going to be long enough to fold and lay across the foot of my queen sized bed. Working on it for a few hours one night a week, it's only taken me a month or two. Granted, I bought way more than I used from my stash, but I had some nasty Red Heart acrylic in a beautiful color and just integrated it in one row segments in the pattern.
My advice: blanket. But you'll end up buying a lot more yarn to finish it lol
My own stash is one of those 3 drawer plastic thingies, and then a single drawer to put near it once I clean my room, and then a 3-tiered wire shelf thingy. I'm sure I have more around, but I'd say it's a small-to-medium stash compared to what I know most serious crocheters have got.