a friend of mine designed some wooden rails for an indoor funpark, he flattened the logs with an electrical wood planer...
^^you should rent one, a makita or hitachi, a quality one because the cheaper black and deckers or bosch are garbage and will break down or jam before you've finished 1 log
with my makita i finish a 12ft log in under half an hour...
sliding it works, but trust me on this, especially for skiers, it's better to put an HDPE sheet on top, because your edges will catch sooner or later.....
if you refuse to go for hdpe, you should finish it off with some lacquer (sp)
as many layers as you have time/money for and the would will absorb...
it hardens the wood, makes it weather resistant, makes it flatter, smoother and thereby better to slide...
(after 1 layer take a sandpaper and brush over it to even everything out, then apply following layer, and so on ...)