Some ideas...
Self-care - mani, pedi, haircut, massage, a reflexology session, etc.
Retail - books/movies/music, household items, personal accessories, etc. Things you might otherwise not buy.
Hobbies/recreation - doing something athletic for the first time especially if it wasn't an option at a higher weight, going horseback riding, taking a dance/exercise/yoga class or getting a gym/yoga studio membership, going to see a movie or a show
Travel - a day trip, a longer trip to celebrate a larger goal
Jewelry - a ring or necklace to symbolize the journey and new beginnings