Dnes bylo klidně.

I kind of don't know how to explain... but you can write: "Dnes byl klid" or "Dnešek byl klidný". And you cannot write: "Dnes bylo klidně" or "Dnešek byl klid"
You could probably use "Dnes bylo klidné" but it sounds really awkward and weird

Večer jsem šla se třemi kamarády.

Dvěa z nich už jsem už znala.

In the previous sentence, you used the word "kamarád" which is masculine gender and means male friend (is used for a group where is at least one male person). Therefore you cannot switch to feminine gender in the following sentence (dvě kamarádky X dva kamarádi). I you were with three female friends you need to write: "Večer jsem šla se třemi kamarádkami. Dvě z nich jsem už znala)

Měli jsme typické belgické jídlo a pití.

Same as above. If there was any male friend, this sentence is correct. If there were only females it would be "měly" instead of "měli"

Potom jsme šli do hospody, kde jsem si objednala horkou čokoládu.

I suppose I ordered it only for yourself (only one cup) so you use the relative pronoun "si" to make clear the drink was for you. When you do not use it, it means that you have ordered beverages for everyone and paid for them.

Na závěrKe konci byl dnešek fajn, ale vlastně preferuju chodi vyrazit si jedem člověk (go out with one person at a time)n s jedním člověk.

"chodit s někým" can be (by word) translated as go with someone, but in Czech it is more frequently used as phrase for dating someone.