Certainly! Here are the rewritten sentences:

1. Nha Trang is smaller than Hà Nội city.
2. Life in Da Nang is less expensive than life in HCM City.
3. Da Lat is less exciting than London.
4. The Huong River is smaller than the Me Kong River.
5. I am taller than Mai.
6. My sister is younger than me.
7. This bag is cheaper than that bag.
8. The country is quieter than the city.
9. Mai's house is larger than Tuan's house.

I hope this helps!