The two sentences without mistakes are:
5. We will have a lot of fun.
8. You should bring a hat because it might be sunny.

Here are the corrections for the other sentences:
1. We **have been** friends since I was 6.
2. I **have known** him a long time.
3. **Have** you heard about the concert?
4. We **have been** here since Friday.
6. I **will see** you at the party then.
7. Tomorrow we will go to the cinema. (This sentence is correct as it is.)