I'd recommend selling some stuff.
Seriously, just go find some things in your basement and sell them on Craigslist. Buy some other stuff and re-sell it. Have some extra concert tickets? Flip those and pay attention to the way the value changes as you approach the concert date. Find something on sale at Target? Buy one and flip it on eBay.
The point it to make sales a "game" for pocket money. You'll never make a ton, but you'll gain valuable experience about writing marketing blurbs, interacting with buyers/sellers, negotiation, the time value of money and products, etc.
As an example: there is, right now, a perfectly working portable air conditioner on Craigslist for free. If I felt like picking it up and storing it until summer I could easy get $100. The seasonal inverse is true for Christmas trees.
...etc etc etc...