I think some airlines will let you redeem awards. However, 15,000 miles is not enough for most awards. I wouldn't hesitate to donate miles.

I wonder if you can find out which charity receives miles regularly. You can then write them a letter to them and NW asking if miles can be donated to them in your mother's name. NW will probably refuse a designated donation but the charity may write you a thank you note, if that helps you remember your mother.

See NW's website to see if there are any magazines or anything else you want.