Just a guide for the location of the generators and cards.
The order in which you get these doesn't really matter.

Generators:
1. Is in the gravel-gray area near spawn. Bottom Right Corner. Should have a little combine booth next to it.

2.The next one can be found inside a shack in the swamp area.
(You'll need to break a wood panel to get inside of it.) - To the Right of the Combine Tower

3. Is underneath the Rebel Town, in the wooden support beams. On a catwalk, there's is a metal panel jutting out, break it and the generator is inside.

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Cards:
Zombie - Inside the Headcrab Guys House, finish his quest to find it.

Fast Zombie - Inside a Shipping Container in the Swamp, swim to enter.

Poison Zombie - White House w/ Blue Roof, it's in the gravel area near spawn, inside the Poison Zombie's room.

Headcrab - Inside a building near a truck w/ barrels/ near the area where the road has opened up. The road is near the building with the giant drink on top of it.

Rebel - Inside an enter-able shack on-top of the Rebel Town.

Combine Soldier - On the Combine Tower, in a Rebel Hideout Area (Look for the Lambda)

Antlion - Across a bridge, near the Cyantine city sign, it's very close to the first generator.

ERROR - Inside the giant TV store, in the backroom of the first floor. The store is close the to the Combine Wall and the Rebel Town, it has a broken orange sign.

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Secrets or Easter Eggs:
(That I could find)

Hiking Trip: Inside the wooden house in the lake of murky water, you can meet the buddy that joins you in Hiking Trip. (Both of these maps are made by the same author.)

Torment(?): When getting the generator in Rebel Town, as I call it, there's an Umbrella-Man Graffiti that glows red. If you walk into it when glowing red, you'll be taken into this red graveyard with upside crosses. You're trapped, take fire damage, and eventually die. Oh, and Grigori laughs at you.

Dev Room: Getting all the cards, rewards you with an empty dev room, congrats. Once all the cards are obtained a small cutscene will play, showcasing the door inside the Cyber Cafe (AKA the place with the Vortigaunts and Rebels.)

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Anyway, that's all I found. Have a good day, afternoon, or evening; whenever, wherever, or whoever you are!
Sorry if any bit of this was unclear, that's my fault.