Here are the sentences with the mistakes underlined and corrected:

1. Many people think snorkeling isn't **much exciting** as mountain hiking.
**Correction:** **as exciting**

2. I think visiting a cave is **more expensiver** than a beach.
**Correction:** **more expensive**

3. Mount Langbiang is **much far** than **love Valley** from the city center.
**Correction:** **farther**; **Lovel Valley**

4. I think Mount Son Tra is **as more scenic** than other mountains.
**Correction:** **more scenic**

5. I find traveling to CON DAO island much more relaxing **as** traveling to other **island**.
**Correction:** **than**; **islands**