#1 is good. In #2, “Have you ever HAD chickenpox? If not, you’d better NOT come today because my children FELL (or, ARE) sick.”
In #3, it’s OK but “finally clean the house” would sound more natural. #4 is good. In #5, “. . .I hope that city IS as beautiful as I imagine.” Also, if you mean to say that it will be the best holiday every only IF the city is as beautiful as you imagine, then you should include some some kind of causal language. Example, “If that city is as beautiful as I imagine, then I’ll have the best holidays ever.” Final thought: in the US, we generally “vacation” rather than “holiday” to talk about a pleasure trip. “Holiday” is usually reserved for a special day in which most everyone has off of work, such as Independence Day (July 4), or for a religious holiday (holy-day) like Easter. Also, if we do use “holiday” to describe a vacation, we usually use the singular. “The holidays” in the plural, in US English, generally refers to Thanksgiving and/or Christmas: “My children will be visiting for the holidays.” But in British English, I believe that the use of “holiday” or “holidays” may be different than in the US.