After moving house recently and seeing just how much I'd been hoiking around with me in terms of makings, I have promised myself I will not buy any more wool/yarn until I have used up what I have here.

Scarves are simple and enjoyable to make, as it's just a great big oblong and you can use different combinations of stitches to get different textures*. I've specified this as a child's scarf because when I started I didn't want to aim too big and end up with some half finished nonsense, and also because I know some children who I will probably give these scarves to. You can scale this up however you see fit, but be aware you'll need a LOT more wool than you might first think.
This project is ideal for using up leftover balls of wool. You will need at least 100g in smaller pieces, plus 100g of a (continuous) finer yarn to hold it all together. I had a couple of big cones of wool which I got from charity shops so will be using these up, along with all the offcuts.

Start off by sorting the wool out into piles which match or clash in ways that are pleasing to you. Consider whether you want the final pattern to be random, neatly striped, perhaps in a rainbow. I went with "random". You will want to get colours which will all go well with your fine yarn.
TIP!
It's a good idea to use wools and yarns which are made of similar fibres so that they'll all shrink at the same rate in the wash. One way of getting around this is to pre-shrink the yarn by washing it in the washing machine (put the balls in a mesh bag to stop them getting too tangled up with themselves and your other washing), or you can check whether a label-less yarn is synthetic or real wool by taking a small amount and (safely) burning it. If the yarn melts, it's most likely acrylic or some kind of artificial blend; if it burns and smokes, it's a natural fibre.

Now cast on, with your first small ball of wool doubled up with your continuous piece. I used ribbing stitch (knit one, purl one) with 20 stitches on 6 mm needles. And then I just knitted, for ages (for a decent scarf you'll want it to be at least 1.5m long). At the end of each of the smaller balls, I would leave around 3 inches of the new colour dangling on the reverse side, and then pick it up into the knitting on the way back. This makes the colour change more gradual as well as spreading the bulk around so the scarf isn't full of lumps.

After casting off and sewing in the loose ends, give your scarf to a child (or keep knitting then give it to an adult - the one pictured here has already been claimed by my partner...) Or keep it for yourself and post a photo in the comments!
*I am not going to attempt to teach anyone how to knit here, mainly because I'm self taught and wouldn't want to pass on
any of my bad habits. There are some great tutorials on youtube and elsewhere that would do a much better job.