What are other non-surgical ways to add volume to a part of the face aside from fillers?

You are correct that fillers are a proven, safe way to increase volume of the face. You may consider Morpheus8 which combines microneedling and RF technology. Though it doesn't add volume, it does improve overall skin tone and texture.

it is hard to advise which would suit you best without a photo, but the Instalift which uses dissolvable sutures to lift and volumize can offer dramatic results. The dissolvable sutures also stimulate your skin to make its own collagen and provides lasting results for approximately 2 years. The Thermitight and Ulthera are 2 other great methods for achieving skin tightening and facial contouring, and I will choose the best option depending on the needs of the patient. Best, Dr. P.

Thank you for sharing your excellent question.  Alternative treatments that may give you results that you are interested in include fat grafting and facial implants that can add volume and shape, while chemical peels can help tighten and improve skin texture.  Hope this helps.