The few lockbox type safes I've found have always either been empty or paperwork. Have only found a few coins myself. Mostly wheat cents and/or world coins. No silver coins or anything like that yet. Those are what I would like to find since I collect coins.
Your chance of coins and jewelry is higher IMO then finding that 1k+ unique item or even guns. Many people have collected coins at one time or the other. Or their children have started a set. Some of the modern mint sets have gone down in price (unless is the silver proof set), while others have gone up. Search those jars of coins that you find. I pull out every wheat cent, etc. that I can find. Roll of common wheat pennies can go for $2 or more. Find a key date and you have a penny that can be worth $10+.
If your area is like mine even world coins that really don't have much worth can be sold for $.25 - $.50 each. Depending on the country, design, etc. Most world coins are not silver...even the older ones. Many people like to collect by the design (aka dolphins, birds, etc.). Or they collect by the country. Kids see them and just think the look is cool. There has been some days at the flea where 1/2 my profit has come from the $.25 - $1 world coins and paper money I've bought from ebay for my own collection.