How can I use leftover 6 inch wood circles?

You can use them to make coasters like the Navage Patch did - https://www.thenavagepatch.com/diy-log-slice-coasters/ or here's a few more ideas you could try - https://www.countryliving.com/diy-crafts/g2628/wood-slice-crafts/

Hi Ashley, You could put small tiles or the glass beads that you can get cheap at dollar tree and make trivets. You could put 3 together and make snowmen or use 1 and make a santa head. Here is a link on cute little painted wreaths.

https://princesspinkygirl.com/wood-slice-painted-wreath/

You could stencil or woodburn some Christmas ornaments.

Coasters or trivits. Use a wood burning tool to customize. Who doesn't love a personalized trivits?

Holiday decor is another option, and Valentine's day will be here before we know it . . . Paint cute sayings and give as gifts.

Here are some Christmas coaster designs I did on cork circles, you could do something similar to these, and hang them as ornaments if your circles are too big to be coasters https://www.justalittlecreativity.com/2014/11/diy-decorative-coasters-bicmerrymarking-challenge.html

Donate them to an elementary school especially one located in a less advantaged area. Teachers will put them to good use as arts and crafts materials, for the children to create custom painted ornaments or similar. More often than not, there is just no budget for these kinds of resources.

Hi Ashley. I'm Cindy. You could string them together to create a message. For example, Merry Christmas, or Happy Valentine's Day, or Happy Birthday. You could paint or burn the letters on each disc. Make them so that they could be hung. Good luck Ashley and Happy Thanksgiving.