Je traduirais par:

1. Qu’il fait beau aujourd’hui !
2. Tu ne sais pas à quel point il peut devenir sérieux.
3. Tu n’imagines pas tous les ravages qu’il peut provoquer.
4. Quel bon voyage !

Main question

Actually, most of the examples you gave translate almost as is.

How beautiful today is !
Qu’il fait beau aujourd’hui !

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You don’t know how serious he can become.
Tu ne sais pas à quel point il peut être/devenir sérieux.

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You can’t imagine how much havoc he can cause.
Tu n’imagines pas les dégâts qu’il peut causer.

---

What a great trip!
Quel bon voyage !

Side question

You do shorten que il as qu’il, always. This is difficult to articulate otherwise. When you have a pronounced e before another vowel sound, it always disappears.

_{I suggest you don’t accept this answer yet as it doesn’t meet my own quality standards. Either somebody will give more explanation, or I’ll come back.}

Well, I’d first answer your side question: you have to shorten it. Que il is not correct. You have to use Qu’il.

So let me correct your different sentences:

1. Qu’il fait beau aujourd’hui !
2. Tu ne sais pas à quel point il peut devenir sérieux.
3. Tu ne peux pas imaginer le chaos* qu’il peut causer.
4. Quel beau voyage !

*I am not very sure of the meaning of havocs but you can probably translate it by le chaos or la confusion or les dégats.

Regarding sentence 2: how serious means how much serious which in French you would translate by à quel point. A closer English version would be You don’t know to what point (or to what extent) he can be serious.

Jumping to exclamations like your examples #2 and #3, where you want include the notion of “you don’t know how/to what point …,” I’d used:

“Tu [ne] peux pas savoir/imaginer … comme (ou à quel point) … (#2)il peut devenir serieux/(#3)il peut poser des problèmes!/il peut foutre le bordel!”

Back to #1, I’d say “Qu’il fait beau!” or « Qu’est-ce qu’il fait beau! » (I’m pretty sure that the « que il » would [nearly ?] always elide/contract to « qu’il » [because of ‘il’ starting with a vowel], whereas “que tu” [consonant] would not elide: “Que tu es belle/beau! ” or « Qu’est-ce que tu es beau/belle! »)

« LA GRAMMAIRE FLE POUR ÉTUDIANTS FINNOPHONES » covers pretty well French Conjonctions ou locutions exclamatives, although I don’t see the use of « Quel/quelle » mentioned, which is what I would use to say your #4 « Quel voyage super. »