Coin stacking is a hobby that was brought to my attention by the website of Mitch Fincher, a Civil engineering student. This hobby involves using coins to build structures. Believe me, it's less boring than it sounds.
Coin stacking is probably something you've already done, indirectly. You stack coins on top of each other to create a structure. There are techniques for creating domes, pillars, spirals, and other things in addition to making the ever-so common pyramid.
For instance: the technique for creating bridges. The following structure illustrates what is needed.
You see, the bridge looks pretty flimsy. This is because there is an 8 penny span from one support to the other. This is very difficult to achieve, and as you can see, the builder needed to weigh down the top significantly for the pennies to have enough support to span that distance.
In conclusion, Coin stacking is a very interesting, time consuming, entertaining little hobby that is certainly offbeat. Best of all: It's practically free! You don't actually lose any money, because you can just spend the money in the structure later.