8/3

Here are the corrections:

Original Sentence: Today is the day my family decided to go to the valley. 
Correction: Today was the day my family decided to go to the valley.
Explanation: Changed to Today was the day to maintain past tense consistency throughout the narrative.

• had some good breakfast: Changed to had a nice breakfast for smoother phrasing.
• at our go-to restaurant near the valley and went to the valley: Combined into at our favorite restaurant near the valley and then went to the valley to avoid repetition and make the sentence more fluid.
  
  Original Sentence:  I had fun playing in the water, but suddenly my water gun was broken.
  Correction: I had fun playing in the water, but suddenly my water gun broke.
  Explanation: my water gun was broken: Changed to my water gun broke for a more straightforward and natural expression.
  
  Original Sentence: However, it was fun just floating around in the water by myself with a life jacket, so I played like that most of the time.
  Correction: However, it was still enjoyable to float around in the water by myself with a life jacket, so I did that for most of the time.
  Explanation: However, it was fun just floating around in the water by myself with a life jacket, so I played like that most of the time: Simplified to However, it was still enjoyable to float around in the water by myself with a life jacket, so I did that for most of the time for clarity and conciseness.
  
  Original Sentence: We had some dinner at the restaurant in the valley after playing in the water.
  Correction: After playing in the water, we had dinner at a restaurant in the valley. 
  Explanation: We had some dinner at the restaurant in the valley after playing in the water: Rearranged to After playing in the water, we had dinner at a restaurant in the valley for better flow.
  
  Original Sentence: I want to buy a new water gun and go to the valley again.
  Correction: I want to buy a new water gun and visit the valley again.
  Explanation: go to the valley again: Changed to visit the valley again to avoid repetition of the phrase "go to the valley."

Final Corrected Version:

Today was the day my family decided to go to the valley. We had a nice breakfast at our favorite restaurant near the valley and then went to the valley. I had fun playing in the water, but suddenly my water gun broke. However, it was still enjoyable to float around in the water by myself with a life jacket, so I did that for most of the time. After playing in the water, we had dinner at a restaurant in the valley. I want to buy a new water gun and visit the valley again.