We recently hosted a kids night with a treasure hunt theme.
The kids (aged 5-6) had a blast completing a series of challenges, all leading to a final surprise!
Here's a breakdown of what we did:

Program starts right before sunset with a BBQ (Hot dogs and corn on the cob), then as it started getting dark (We did this in late december so that was around 6pm)

Challenge 1: Target Practice with LED Arrows

• Each child was given a slingshot and a projectile “arrow” with blinking LED tips.

• The objective was to land the arrow inside a designated target.

• All children had to successfully hit the target before moving on to the next challenge. So they started helping and cheering for each others.
  
  After Challenge 1: Hint 1 of the treasure hunt was given. (In this case I had the layout of our plot and I crossed/eliminated out a section, could have been more creative)

Challenge 2: Flashlight Communication

• The children were given mini flashlights and used them to spell out letters to the a receiver positioned ~40 meters away.

• They had to figure out how to do it (connect the dots style) and then coordinate to position each ‘dot’ to make a letter.

• The receiving group, equipped with a walkie-talkie, had to guess the letters being signaled. We did 3 letters.
  
  After Challenge 2: Hint 2 of the treasure hunt was given.

Challenge 3: Laser Mirror Maze

• Four mirrors were strategically placed in the yard.

• The children had to work together to direct a green laser beam, using the mirrors to reflect it onto a target at the end.
  
  After Challenge 3: Hint 3 of the treasure hunt was given. Which also eliminated a section of the map and a simple riddle “I am surrounded by wood on all sides” so they could find the treasure in the backyard gardening tools shed.