shimple one is 'shu-matsu dou deshita' (週末どうでした) But if you want to ask what friend was doing on weekend, you can say ' shu-matsu nani shite mashitaka' (週末　何してましたか）

I think 'げんき？' or 'ちょうし　どう？' are better. Your friend may say 'げんき' (I'm fine.) or 'まぁ　まぁ' (so so) or ' とくに' (nothing special) or something about yesterday etc...

I'm not sure, but if you ask about when you use 'subject + ga' and 'subject + ha' , I can advise some. When you want to enphasize WHO did(will), you can use 'ga' (e.g watashi ga...). For example, Who ate?(dare ga tabetanodesuka) I ate.(watashi ga tabemashita) And When you say 'in your case' or ' subject = noun', you can use 'ha'(e.g watashi ha...) For example, I won't go there.(watashi ha ikimasen)←somebody may go there. I am a sutudent.(watashi ha seito desu) I=a student. We are using them like this.

夏休みは外国です（ご）（し）たい。　だと思います。

this mean is usualy a woman who shows next game number on the boxing ring between game asnd game.