日常 - Routine

上星期日我们的城市有一个马拉松。- Last Sunday there was a marathon in my city.
阳光明媚，天没有云，不热。- The Sun was shining brightly;, there were was no clouds in the sky;  it was not hot.
我没参加马拉松，我去有野餐。- I didn't join the marathon, but I went to have a picnic.
我们做简单的菜：香肠和几个沙拉。- We cooked made simple dishes: sausages and salads.
下野餐我开车去买东西。- After the picnic I drove to do get some shopping done.
下午我有一个中文课。- After lunch In the afternoon,  I had have a Chinese lesson.
晚上我看很有意思的电影。- In the evening I watched an interesting a meaningful movie.
天真忙阿！ - It was Such a very busy day!

Very good in your translation.  Except for a few minor things here and there, you are doing very well.

日常 - Routine

上星期日我们的城市有一个马拉松。- Last Sunday there was a marathon in my city.
阳光明媚，天没有云，不热。- Sun was shining brightly, there were no clouds in the sky, it was not hot.
我没参加马拉松，但我去有去野餐。- I didn't join the marathon, but I went to have a picnic.
我们做了几个简单的菜：香肠和几个沙拉。- We cooked simple sausages and salads.
下野餐之后我开车去买东西。- After the picnic I drove to do shopping.
下午我有一个中文课。- After lunch I had a Chinese lesson.
晚上我看了一场很有意思的电影。- In the evening I watched an interesting movie.
那天真忙阿！ - It was a very busy day!

btw, are you living in Sydney?

日常 - Routine

上星期日我们的城市有一个马拉松。- Last Sunday there was a marathon in my city.
阳光明媚，天没有云，不热。- Sun was shining brightly, there were no clouds in the sky, it was not hot.
我没参加马拉松，我去有野餐。- I didn't join the marathon, but I went to have a picnic.
我们做简单的菜：香肠和几个沙拉。- We cooked simple sausages and salads.
下野餐（后）我开车去买东西。- After the picnic I drove to do shopping.
下午我有一个中文课。- After lunch I had a Chinese lesson.
晚上我看很有意思的电影。- In the evening I watched an interesting movie.
天（那天）真忙阿！ - It was a very busy day!

My comments:

1. about the word "after" 

This word can often be translated into "……后”

Eg1. After the picnic, 野餐后

Eg2.After lunch 午饭后（of course, u can use "下午“here)

Eg3. After having dinner, I watched television yesterday. 昨天，吃完晚饭后，我看了电视。

2. about the word "天“

When "天” is used separately, it usually equals to "sky"

Eg. The sky is really blue. 天真蓝.

3. about the word "day"

Today——今天

Yesterday——昨天

Btw, I can easily understand what u wrote in Chinese, you write well!

日常 - Routine

上星期日我们的城市有一个马拉松。- Last Sunday there was a marathon in my city.
阳光明媚，天没有云，不热。- Sun was shining brightly, there were no clouds in the sky, it was not hot.
我没参加马拉松，我去有野餐。- I didn't join the marathon, but I went to have a picnic.
我们做简单的菜：香肠和几个沙拉。- We cooked simple sausages and salads.
下野餐后我开车去买东西。- After the picnic I drove to do shopping.
下午我有一个中文课。- After lunch I had a Chinese lesson.
晚上我看了很有意思的电影。- In the evening I watched an interesting movie.
那天真忙阿！ - It was a very busy day!

日常 - Routine

上星期日我们的城市有一个马拉松。- Last Sunday there was a marathon in my city.
阳光明媚，天没有云，不热。- Sun was shining brightly, there were no clouds in the sky, it was not hot.
我没参加马拉松，我去有野餐。- I didn't join the marathon, but I went to have a picnic.
我们做简单的菜：香肠和几个沙拉。- We cooked simple sausages and salads.
下野餐之后我开车去买东西。- After the picnic I drove to do shopping.
下午我有一个中文课。- After lunch I had a Chinese lesson.
晚上我看了很有意思的电影。- In the evening I watched an interesting movie.
那天真忙阿！ - It was a very busy day!

很不错!