I tied my quilts for the first 10-20 years. Basic craft yarn makes big holes in the fabric as it goes through double even if you tie it single (hopefully that makes sense) and can be hard to push through as well as needing a huge eye in the needle, but there are some yarns I would use. At this point in my quilting, I'd go with some sort of embroidery thread and I would go at every 9-patch block intersection. I've seen a lot of tied quilts that are one stitch in the middle square. Buttons can be a fun way to tie a block, if you don't have a button collection they have cheap craft button collections, but they make the quilt top only, you do not want to lay down on a bunch of buttons.
Keep in mind, that if you do a simple tie at the corners, when you get your machine back you can use the ties as basting and quilt it all down again and remove the ties.