星期六，下雨（if you want to write it in a diary form, you only need a noun here）

今天是星期六，所以我想休息一下。
1. There better be a reason when you bring up the special day, so add “所以” will be more natural.)
2. “我休息一下” is not a complete sentence, you need to add grammatical tense here, like
“我休息了一下”(past tense)
or "我准备休息一下" 、 “我想休息一下”、“我正要休息一下”(future tense)
or "我正在享受这段休闲的时光"(present tense)

但是，虽然现在是澳大利亚的夏天，今天下雨、云很多、温度不高、只有二十度。所以我不会在海波步行，要在电脑学习中文。什么样的周末？
现在是澳大利亚的夏天，虽然温度不高——只有二十度，但是今天云很多，还下着雨，所以我没有选择在海滩上散步，而是坐在电脑前学中文。
1. 但是、虽然 share the same meaning, it would be cumbrous when you gather them together, it's like "He's eating a big large apple".
2. When you use an adversative to lead the preceding half sentence, you mean its the reason of something that you will mention in the latter hafl sentence. So it would be better if you try to emphasize the latter sentence with another adversative word leading it. We have some pattern here, like "虽然……，但是……" this website would be helpful http://baike.baidu.com.cn/view/285476.htm

3.
(1) 今天下雨、云很多、温度不高、只有二十度 。。it's a little bit fragmentary, like a poem or something. This piece"只有二十度" is a supplement of "今天温度不高”,so use a dash here would explain this relationship, the dash here is equal to "which means", like“爱因斯坦——天才” or “说到人类在量子力学领域的财富——霍金……”
(2) And there is a progressive relationship in this four fragments. It will be 温度＜云＜雨. Why？ Consider which is the worst situation. So I change the order here, worst is the last, people like to hear good news first.

4. When you substitute an action with another, again use an adversative, like"而是" .
Chinese adversatives have different subtexts, it'd be better if you try to figure out them one by one. But, there's not many of them, good luck with that!

5. You lost me in the last sentence....

星期六，下雨

今天是星期六，我休息一下。但是，虽然现在是澳大利亚的夏天，今天下雨、云很多、温度不高、只有二十度。所以我不会在海波步行，要在电脑学习中文。什么样的周末？

今天是星期六，我可以休息一下。现在澳大利亚虽然是夏天，但是，今天下雨，云很厚，温度也不高，只有二十度。所以，我不想去海滩散步，而是要在电脑前学习中文。这样过周末怎么样？

星期六，下雨

今天是星期六，我想休息一下。但是，虽然现在是澳大利亚的夏天，但是今天下雨、云很多、温度不高、只有二十度。所以我不会在海边波步行，我要在用电脑学习中文。这个周末怎么样？什么样的周末？

星期六，下雨

今天是星期六，我想休息一下。但是，虽然现在是澳大利亚的夏天，但是今天下雨、云很多、阴天下雨，温度不高、，只有二十度。所以我不会在去海波边步行散步，要在电脑上学习中文。这是个什么样的周末？!