It's pretty obvious, but just in case, the Egged achievement is awarded for finding any of the three available easter eggs. The following eggs are in order of how fast/easy they are to get.

If you're only looking for the achievement, I suggest you grab this egg and call it a day. It is relatively easily accessed from the beginning of the game - you just need to be able to serve coffee.

   
If you check out the flyer stand next to Patricie's counter, you can find one that has a set of reviews for the cafe on the back. One of these reviews is from Sonny Joon.

If you're too lazy to check it yourself, Sonny says he had:

• 1 shot of espresso
  
• 2 sugars
  
• 1 portion of steamed milk

All you need to do is make this coffee, and you'll get the egg!
Congratulations, you're now a third of the way there.

To get this egg, you'll have to have discovered the alchemist lab, while also not having yet opened the locked cabinet. Once you meet those criteria, it's pretty easy to get. The egg in question:

    
All you have to do is go to the locked cabinet thing in the alchemy workshop. Instead of solving the puzzle correctly, align any three of the same symbol, and you'll be awarded the egg!!

2/3. Awesome.

As much as I enjoy cats, this is by far the most tedious and annoying easter egg to acquire.

   
The first thing you'll need is to find Leo's cat, after which it will start to appear in various different places. Specifically 10 locations. You will need to find the cat and pet it in all 10 areas, in order, to get the egg.

It appears in the following locations (chronological order):

• On the floor at Leo's office (right after you free it)
  
• On the circular bench next to the statue outside
  
• In the Aparat side of town, by a truck in a corner of the area (to your left-ish when you enter)
  
• In the room between the museum and jewellery shop, behind one of the pillars
  
• On the floor in Marek's shop, by a wall
  
• On a bench outside (castle square) by the food stalls
  
• On the floor in the underground tunnels (can't help with directions my navigation skills are poor)
  
• In the cafe, by the wall that has images from previous ND games
  
• Next to a truck in front of the clock tower on the Aparat side of town
  
• By the knight armour exhibit in the museum

After the final petting, you'll have gotten the final egg! Woohoo!

I would love to muse about the downfall of the franchise, the difficulty with which I watch HeR make progressively worse decisions, while still being loyally devoted to the Nancy Drew series, but you'll probably have seen enough of that already in the reviews.

Instead, let us come together to enjoy these easter eggs and look forward to the days to come.

May the Egg be with you.