One of the unique keepsakes that people use are challenge coins. They have a long history and come in a wide variety of types, but with improvements in technology they can be produced in a large price range. They can be simple metal coins that have been painted to coins made of precious metals and die cut that have been polished to a mirror finish. Which one is used depends on what they are being made for.
Perhaps the oldest and most common use of challenge coins is in the military or other fraternal organizations. While they can and are made for each branch of the service the more meaningful ones are the unit coins. They are somewhat of a team building tool as only members of a particular unit would have one like it. Some units will even play games with them. When the unit goes out the person who forgot to carry theirs has to buy around. Another use they have are for servicemen who pass a certain training like boot camp or jump training.
The military also uses them in a commemorative function. Sometimes they are used by for survivors of a large costly engagement or used by the general public to remember it. Generally if they are made for both the military and public they will be different.
In that same spirit many companies have adopted the use of challenge coins as well. They might be given to all members of a team or they might be given to everyone who passes a certain challenge. That might be something like a team building retreat or maybe a certain educational challenge.
Nowadays many people use challenge coins for personal milestones. Some people use them for their wedding and let guest keep them as wedding favors. Other times older couples will use them for large events like a 50th wedding anniversary.
Because of advances in technology the printing prices have dropped. This make them very suitable for other uses such as advertizing. Because a coin is permanent they can be a much better advertizing media than other things. Their permanence also means they are more likely to kept rather than just thrown away.
Challenge coins are unique object that can be customized to any situation. They can be die cut from silver for important events or painted to make an effective advertizing tool. They might be for members of a military unit or for members of a wedding party. No matter what you use them for they will be around for a very long time.