Atticus might put something related to the law, perhaps the US Constitution in a time capsule along with the transcripts from the trial of Tom Robinson. These items represent Atticus' affinity for and dedication to a sense of justice. Friar Lawrence might include his own law book, the Bible, as representation of the source of his own inspiration.
Friar Laurence might well make a confession of his sins of pride and abandonment of the children under his spiritual care, and include the marriage license of Romeo and Juliet. Perhaps, Atticus might write an analysis of the Tom Robinson trial a la Vincent Bugliosi : Outrage: How Bob Ewell Got Away with Perjury and cost a Mockingbird His Life.
Being a sentimentalist as well as spiritual, Friar Tuck would no doubt put in the vial that led to the deaths of Romeo and Juliet, and maybe a copy of the Duke's decree banning sword fighting. Being a father and moralist, Atticus might put in a pair of Scout's and Jem's baby shoes, and maybe a pencil drawing of Tom.
This is an interesting question for a class that has just completed studying both To Kill a Mockingbird and Romeo and Juliet. I would suggest that Atticus include a court transcript of the Tom Robinson trial that would allow future generations to see how some juries of the past were influenced more by the color of a man's skin than the facts. Certainly a photograph of both Atticus and Friar Laurence standing side-by-side would be a one-of-a-kind remembrance. A photo (or artist's rendition) of Romeo and Juliet together along with Jem and Scout would illustrate the change of their very different times. Most importantly, I would hope that the friar would include a sample of the antidote that would have saved Romeo from his deadly poisoning, as well as the dagger that Juliet used to kill herself. Hopefully all of these items would show the dangers of rash behavior to those who viewed them in the future.