Do I have to put "doing" ? No... What do you look forward to at the weekend ? does this sound right? Yes... Without "doing", the questioner expects a simpler more general answer: - I look forward to a bit of peace and quiet. I look forward to just sitting around and maybe reading or watching movies. I look forward to being with my family. With "doing", the questioner expect a little more detail about the activities that you would be doing at the weekend: - I look forward to playing with the children. I look forward to to doing barbecues and hanging out with my friends. I look forward to going on hikes in the mountains with my family. So, you see, there is not a big difference between these two questions. They are both good.👍

Yes its better to put what are you looking forward to on the weekend

It is correct very correct but I would say " What are you looking forward to do over the weekend? “