Local foods, cookbooks, travel books, CDs and movies about or filmed in the area....
For me it's usually Loveless Cafe cookbook, Jack Daniels's cookbook and Tipsy cake, Tennessee Tea Cakes, Goo Goo clusters, Moon Pies, Martha White or White Lily biscuit and cornbread mixes as a base and then I have picked up travel guides and books about the region. Sometimes I have put in a bottle of TN wine or Jack Daniels wrapped up in plenty of bubble wrap and Loveless Cafe's ham and bacon that is vacuum packed and doesn't require refrigerations - never had any of it get confiscated or bottles broken. If I really want to give them an idea of good old Southern foods, I put in a can of blackeyed peas, turnip greens and Brunswick stew. Just look in your pantry and think about it and you can come up with a ton of ideas. Put it all in a handmade in TN basket, maybe tuck in a locally made handpainted mug or bowl of some sort with a note explaining each item (I even do this for other gift baskets - it's what gets the most compliments - the fun stories about why we chose each item).
OH forgot, this may be the most fun part for them - a CD of regional music such as a good Grand Ole Opry sampler or bluegrass sampler for my region. Or copy of a movie made/about TN such as WW & The Dixie Dancekings, Beverly Hillbillies series,etc. Just make sure to send in a format that works on their DVD players but they got a real trip out of the Beverly Hillbillies and Andy Griffith shows we sent them.
Don't worry about what you think they can get locally or what her hubby can pick up - pick some things unique to your area especially things you like, include a little note explaining each item and don't worry about it - they'll love anything you bring them and it's the thought that counts!