Welcome to Gaia! ::

Depending on the type of yarn perhaps for filler for something like these dryer balls?
Or stuffing for anything that needs stuffing?.

There's a list here. most involve knitting or crochet but one involves making new yarn balls. I think that could be cool for some projects if you plan to make things like afghans or scarves... I could even see an interesting skirt or sweater made from such yarn if done right.

Wrapping twigs is also interesting.

You could always save them to tie quilts with. It would be really cool to see a quilt with multiple colors of yarn knots.

Another idea is also to donate them to a school. They can always find some way of using the bits and pieces.Or place some outside in the spring when birds are building their nests. They should end up using them.

If you have paint and an extra canvas or paper you could try string painting.
http://www.cutoutandkeep.net/projects/string-paint

It looks really cool with a several bright colors.