Today I am tired. Today I am tired.

Today tired.ired Today Tired Today

Or "Today I'm Tired"

Today tiredwas tiring. Today was tiring.

Today's club activity iwas very hard. Today's club activity was very hard.

Today's club activity iwas very hard. Today's club activity was very hard.

Unless you were still working on this activity during the time of your writing, this should be past tense

Today's club activity iwas very hard. Today's club activity was very hard.

I am too tired to use my brain hardmuch. I am too tired to use my brain much.

Or maybe "I'm too tired to think"

I'm too tired to use my brain harfor anything complicated. I'm too tired to use my brain for anything complicated.

More natural ending here

I'm too tired to usework my brain hard., I'm too tired to work my brain hard,

Today's sentence iss are very short. Today's sentences are very short.

Since you wrote more than one sentence, you should use the plural. Alternatively, you could say something like "Today's post is very short"

Today's sentence iss are very short. Today's sentences are very short.

Plural since you have more than one sentence in the text

Tso today's sentence iss are very short. so today's sentences are very short.

I'm sur hope everyone will have a good night ! I hope everyone will have a good night!

I'm sure everyone haveis having a good night ! I'm sure everyone is having a good night!

Or if you wanted to wish people well, "I hope everyone has a good evening!"

I'm sur hope everyone haves a good night ! I hope everyone has a good night!