A plug for rollerblades.
They may not exactly simulate Skiing. (Blading is harder) requires better balance and small muscle control (The same thing) If you put in about 100+ hours rollerblading over the summer/ Fall you will show up on the slopes in very good shape. Strong knee's,ankles, and core. You will have exelent balance, and fast reflexes. Who cares if you are not simulating ski technique while rollerblading, besides being a fun (Dangerous) way to get some exercise, you are developing all the strength and balance skills required to be a great skier.
I Blade but not as much as I should