Sure, I'll help you identify the parts that need correcting:

1. Having leisure activities are truly important to the elderly.
- **B. are** (It should be "is" because "Having leisure activities" is a singular subject.)

2. Although she wanted to go to the museum, she decided staying at home.
- **D. staying** (It should be "to stay" to maintain parallel structure with "to go.")

3. Parents are concerned that their kids may be spending too many time on screens.
- **C. too many** (It should be "too much" because "time" is an uncountable noun.)

4. Collecting coins is exciting, but it can also be relaxed.
- **D. relaxed** (It should be "relaxing" to match the adjective form.)

5. Cloud watching sound weird, but Hang adores it.
- **B. sound** (It should be "sounds" to agree with the singular subject "Cloud watching.")

6. For some young people, enjoyment involves sitting in front a computer playing games.
- **C. in front** (It should be "in front of" to be grammatically correct.)

7. His parents are thinking of banning him on using the computer.
- **C. on** (It should be "from" to be grammatically correct.)

8. When you play games online, be carefully when making friends with strangers.
- **B. carefully** (It should be "careful" to be grammatically correct.)

Here are the corrections:
1. B
2. D
3. C
4. D
5. B
6. C
7. C
8. B