Like was said before, Squats & dead lifts...PLUS Calf exercises! Also throw in a lot of core work (abs/low back etc).
Also, if you have the $$, buy a balance board (Or you can use a skateboard deck and a 2L soda bottle filled with water if you are ghetto).
If you have even more $$, check out mountain boarding/long boarding or freeboarding. All are somewhat like snowboarding.
If you are mechanically inclined, build a box in your back yard and some sort of ramp to ride down to hit the box. You can use an old tarp and a lot of soap to make the tarp slippery. Once it's slippery and you have a ramp to ride down to get some speed, just practice hitting your home-made box all spring/summer long.