JOEL - The contents of the corner stone time capsules can vary from one temple to the next. The local future temple patrons usually put things in that are meaningful to them or to their community. Here is a list of some of the most common things that are usually included:
-Copies of the scriptures and Hymn book
-History book and photos of the area and people
-Letters and testimonies of local church members
-Most recent Church almanac
-Recent copy of Ensign magazine
-History and pictures of the construction of the temple
-Pictures of the First presidency and Apostles
-Tools used during construction

The cornerstone of the original Nauvoo temple contained the original manuscripts for the Book of Mormon. But later when they were retrieved it was found that water had seeped in over the years and damaged them quite severely.
These days, a typical LDS temple cornerstone is not really a cornerstone in the classic sense but rather a time capsule at the corner of the temple that is sealed with mortar at the time the finished temple is dedicated. Most large buildings today are steel framed and have no structural reliance on a cornerstone. By contrast, a cornerstone in the old days actually supported the building's foundation and walls. Hence, cornerstone ceremonies were held at the time the foundation was laid. Now they don't happen until the temple is finished.
Usually on some anniversery date perhaps 50 years later, or when a temple is being renovated, the cornerstone time capsule will be opened and the contents put on display and then something more might be added to it before it is sealed up again.