I recently found out that coin collecting can be profitable. Though, it takes some effort to get your hobby to that point.
There are a couple of factors that make coins valuable: material and rarity. Coins that are rare are worth more as historic/collector items. Coins that have significant amounts of precious metals will have a certain amount of value-based solely on what they are made of.
Of course, several coins will have both. They will not only have precious metals but also have historic significance and probably be rare finds too. There are coins so valuable that they are almost too pricey to purchase unless you want to spend lots of money on them.
With that being said, you can stumble upon rare treasures and purchase them before someone realizes what they have. If you can find a rare-enough treasure, purchasing even a single coin can garner you some serious coins (ha-ha!) when you want to sell your collection.
Coins are one of the few hobbies that let you get a good return on your investment if you ever desire to sell them. This is particularly true if you pay attention to what the coins are worth when you purchase them, and you only spend money on good deals. Coins never lose value.
Also, the value of coins usually fluctuates a bit in the market. The market values of precious metals can go up and down along with the economy. Though over time, they typically increase in value, meaning that coins are also a hobby that usually is a solid investment.
If you stick to buying coins that contain precious metals (older coins), then you will always see their value rise over time. Some folks even go as far as to buy coins instead of stocks just because they usually have such real increases in value.