Interesting question and a great topic for further discussion. If you are talking about a time capsule to open years after Atticus's trial is over, there are plenty of things to put inside.
First, I would add all the newspaper clippings of the trial itself. It is incredible to see a before and after comparison of the mentality of the mainstream media and the public within a period of time regarding topics such as racism and prejudice.
Second, I would save pictures of Maycomb. According to Scout, Maycomb changes through her eyes as she grows older. The more she learns about people, the less she recognizes Maycomb. Perhaps a picture of the town will help reflect, in the future, about the changes in perception one experiences as life passes you by.
Certainly I would collect every single element of prejudice that harmed the case: A lynch, every symbol of anger that was expressed towards Atticus, a list with all the names he was called, even the names and last names of every person who has turned against Tom Robinson even knowing that is an innocent man. Those are all the names of shame. These are the so-called "honest, hard-working church-goers" that did not have mercy on a fellow man just because of his color. It is imperative to remember those names because they will be the elders of future generations who would be ashamed to find out what their ancestors once did. That definitely should be placed in a memory box.
However, as I stated previously, this is a wonderful topic for further discussion and my point of view is strictly historical. Someone might have input as far as Atticus's personality, or his family and friends. There are a myriad of things one should aways remember from the past so that it never repeats itself in the future.