Here is the quick of it. My group is playing a pirate themed homebrew campaign where we take turns being the DM (it takes the load off of one person immensly) and we sometimes introduce props for more emersive play. The last session, the players were actually given a rolled up piece of parchment and were told "this will lead you to the sandman (our lv5 boss)." Now in actuality the DM gave us a beautiful blank rolled up piece of nicely stained and aged looking paper and said that I can do whatever I want with it.

So here's the question. 

I have a mysterious piece of paper that will lead me to the boss. What is on it when the crew opens it up?

An interesting one!

Perhaps look into invisible inks. I believe lemon juice is a key ingredient (or perhaps an entire recipe) for one which appears with heat.

If you can, then perhaps draw an island (wherein waits the Sandman) in invisible ink, then draw the route across the island in normal ink. If you can find a different invisible ink, which reacts to UV perhaps, then you could also draw landmarks separately. That way they have 2 ways to unlock what the squiggle on the map means, and if they get both, it becomes significantly easier!

The paper content could be hidden behind invisible script that only wet sand will reveal, indicating upon sticking to the invisible ink the secret location of the Sandman's lair or treasure.

Your heats desire is the only limit to what could be on that paper.

Then again it could just be a blank piece of paper.

Some possibilities:

When a certain keyword is spoken, it becomes a tiny paper construct that physically leads you to the Sandman. The keyword is an answer to a riddle.

Folding it correctly make it act like a compass. If you have anyone in your group that likes oragami, give them a set of instructions to make something with a point on the end (an arrow, an animal with a snout, a sword, etc). The folding instructions are gained through either research or by a social encounter with some wise figure.

Rolling it into a tube and looking through it allows you to see into the ethereal realm, which allows them to navigate a maze, but only the person attuned to the tube can use it. Give one player a map or set of instructions. They have to tell the other players what to do and the players have to follow these instructions. Make the puzzle as convoluted as you wish.

A single, cryptic word. Could be a name a place, an item that’s important to this Sandman. Think: Rosebud.

It’s a pirate campaign. What else could it be but half a map?

1. I clearly crack myself up.

2. I have too much time on my hands...

You tear off a small corner of the paper and pop it in your mouth.

It's been soaked in a high quality hallucinogenic and you have visions which will lead you to the Sandman.

It's a scroll that let's you summon a particular Mechanus construct. You can ask this thing one question. Of course, if you'd like the right answer, you need to know the right question.

It is a recipe for a making biscuits or cookies.  But in the recipe are clues to the location of the Sandyman.

Quote from bobberuchi >>

It is a recipe for a making biscuits or cookies.  But in the recipe are clues to the location of the Sandyman.

 They could be biscuit sablé #shortbread ☺

When the paper is burned it transforms into an ornate magnetic key. Float the key on quicksilver to point to the sandman.

The true name of the npc, written in their blood (taken from a blade that pierced them in a fight).

This gives the target disadvantage on saves for some spells, e.g. dream, and is a requirement for other spells. 

Either way, it’s something that could then send the party off to a wizard to figure out how to use it… do they go to the kindly cleric, evil wizard, or dodgy warlock? What do the wizards ask in return, etc, etc…

Quote from Plaguescarred >>

The paper content could be hidden behind invisible script that only wet sand will reveal, indicating upon sticking to the invisible ink the secret location of the Sandman's lair or treasure.

 Or, since it came from pirates, perhaps sea water?