Just listen and try to respond with respect.

I let them vent

Take them out for coffee and cake/copious amounts of alcohol.

well I tell them It's okay (for others) for my Family/Crewmates I hug them

Vent session and tell them to take an extra long lunch

Depends on the person, or maybe the context/situation? Like one lady I knew loved Mountain Dew and those Reese's animal crackers and so I got her some of them when she was going through it...or one time someone was just feeling alone and needed someone to sit with them for a while, or one guy lost his wife and what he really needed was to feel normal again so when he got back from his leave we took him to the pub and stayed out way too late and called in 'sick' the next day like old times

Listen to them. We bitch to each other a lot.

Nothing, even if I did work, I don't have anything close to the confidence or authority (or even social skills) necessary to try to console someone / lift their mood. Maybe if it were a friend instead of just an acquaintance I could, but even then, I tend to think more like "it is what it is" and that any comforting words would just be empty fluffery.

I could give them a day off or switch their schedule to give them more ease and time to mend things, also I could offer to listen to their sorrows or problems or emotional turmoil if any. Some bad day can be a sign of a deeper root cause. But you only find out by communicating. Also saying nothing and just having an open soul and ear can help and is also communication.

"Yo I'm paying dinner after work today, y'all invited, wanna come?" or "Hey I'm throwing a party at my place tonight, come and bring your friends"

I generally get them a treat during my lunch break and surprise them with it. And if they want to hear me out - give them a quick little pep talk.