How can I thank the person who lent me money?

Besides paying back the money on time. Don't thank anything special this time. If she needs help in the future, help her. Or bring some local specialties when you come back from business trip. Or what is delicious at ordinary times, bring a double.

Sometimes giving charcoal in the snow is more memorable than icing on the cake. As long as you are sincere to her, she can feel it.