If you search “stash scarf” on Ravelry you will fall down a rabbit hole. I could have linked such a search here, but I’m not going to do it, because I don’t want you to go where I’ve jut been unless you’re prepared for it. There’s so much beautiful stuff out there you’re going to be glad you have stash, for once.
There are long, sideways projects like the Snippet Scarf by Becky Herrick and Sharon Maher’s Scrap Happy Scarf. Sarah Core’s Mini Mania Scarf has the same idea, worked with little skeins of sock yarn, but this time in linen stitch.
Anicka Quin at Knitting in the Buff combines a bulky and a finer yarn for fun texture in her Stash Scarf, while Andre Sue used a bit of Noro and some other yarn for a lovely Chevron Scarf. And speaking of Noro, Jared Flood has a take on a 1×1 ribbed scarf using Noro Silk Garden in a few different shades that could easily be adapted to other yarns.
Fun stitch patterns are great to combine with all sorts of different colors of yarn, or a single skein or two you don’t know what to do with. Consider the woven look of My So Called Scarf by Imaginknit (I actually have some of that yarn in my stash!), Lisa Bruce’s chevrony Favorite Scarf Ever, the Entrelac Scarf by Allison LCicero (there’s that Noro again!) or the Morning Surf Scarf by Jackie Erickson-Schweitzer (I’ve made one similar to this). Or take Frankie Brown’s advice and don’t even knit a rectangle; make triangles instead!
And that doesn’t really even scratch the surface. Sometime when you’ve got a lot of time — and yarn — to burn you can do the search for yourself if this doesn’t give you enough ideas!