It is a large mountain of sand and those who go to Sand Mountain can tongue slide, ski, or snowboard just like any other ski resort.
Even though it hasn't appeared with the ski resort in the few appearances it has made, there probably is a boat parking area, as lots of locals and tourists come here.
Role in series
Sandy and SpongeBob slide down the mountain. They pass other guests that are skiing. Two kids are also making a sandman (the sand equivalent of a snowman). In this episode, there is also a ramp on Sand Mountain, despite the fact that it does not appear in any other episodes that Sand Mountain appears in.
"I Had an Accident"
Patrick, Sandy, and SpongeBob are seen sliding on the mountain. SpongeBob injures himself by breaking his gluteus maximus (butt muscle) and becomes paranoid throughout the rest of the episode.
Tales From Bikini Bottom
It is seen in the "Prehibernation Week" segment of the book, where it has the same role it has in the episode.