I once spent a Saturday afternoon designing and creating document specifically for a campaign. It basically looked like some mouldering old parchment with some random, crazy poetic prophesy on it.
It was, in actuality, the cornerstone of the entire campaign.
Hidden in the documents were clues for the party to chase down.
1. If you read the 1st letter of each word in the 1st column vertically, it spelled a message telling you to keep looking further for more clues.
2. The 1st word of each sentence spelled out a clue.
3. The 1st letter of each word spelled out a long clue.
4. The words taken in order of the Fibonacci sequence spelled out a clue.
5. There were 5 strange symbols watermarked on the parchment, which when connected formed a pentagram. The lines of the pentagram crossed certain words, which spelled out a clue.
6. Inside the pentagram's inscribed pentagon in the middle, the words in that section rearranged to form a clue.
7. Drawn in lemon juice (invisible ink which you heat to make visible) was a map, and certain words on the parchment now corresponded to things like passwords, or hints of secret entrances, etc.
8. When folded along the North/South line of the map, the map itself formed another picture, which now looked like a glyph necessary to understand the final room inside the map's fortress.
I made the document all stained, dog-eared, and torn. I soaked it in tea, baked it in the oven, and then soaked it in different tea again. I aged it and messed it up.
I then handed it to the players at the appropriate moment (they found it hidden on the corpse of an old hermit/prophet).
I didn't tell them anything, and it took them a week to realize that the 1st column of letters warned them of hidden clues.
Over the following weeks, they'd pour over the document, trying various combinations, until they discovered every secret -- which would then lead them to the next piece of the campaign.
I swear --- that stupid document got just as aged and worn from the players as it did from me! One of them *always* had it on them, so they could whip it out and pour over it during their spare time, looking for any more secrets.
Finding the map hidden in invisible ink really messed with their heads!