Hi, I'm Dr. Bonk. Does Lucy have any health issues that would limit what she should eat? If not, some good choices are Chasing Our Tails Dehydrated Chicken Breast Dog Treats. These are exactly what they say they are, dehydrated chicken breasts, nothing else! Also, they are made in the USA.
Otherwise, most quality dog food brands, such as Science Diet and Royal Canine will make their own treats. They have a variety of formulas such as light for weight management and hypoallergenic. If Lucy has a sensitive stomach, sticking with treats from the same brand of dog food that she eats usually helps prevent any upsets.
Some other types of treats that dogs usually like are canned green beans, raw carrots (not baby carrots, they contain a lot of sugar), and canned pumpkin. You can even give her pieces of her normal kibble. Dogs consider it a treat if they are given it at a different time or place than their normal feeding.
Whatever you find, try to limit the treats and be sure to adjust her meal amounts to account for the treats. Doxie's are prone to back problems as I'm sure you know, and being overweight is a huge contributing factor. I hope this helps, please don't hesitate to contact me with any further questions. Thank you and good luck!