I agree with you -- its next move would either be down or up. :-)

But each step along the way is random. If the random value is 0.00000 to 0.49999, it will go down; 0.5000 to 0.99999, it will go up. The amount of the move is based on a standard deviation, as is the daily range. Where the close will be within that range is another random variable. I also have a parameter for how much emphasis to place on the prior closing price, compared to the prior day's average.

Then...turn it loose. See what pops up.

You could make it more complex by adding additional "commodities" that are partial market substitutes. That should limit drift away from some mean value. You can add daily limits. In short, have fun with it. See how many chart formations you can find. 

And remember, this is all from random motion. It has no predictive value.