Mount Chilliad is the biggest mountain in the game, on the very north of map. You can get there using a cable car. It's the majestic place from which you have a perfect look on the whole San Andreas, but you can also find here few surprises. On the very top, you can find a parachute and two cross motorcycles. You can ride down from the mountain or jump from it, or make both at once triggering the parachute while being in the air on the motorcycle. What is more, here one of the biggest game mysteries is located. At the final station of the train you find a mysterious painting on the wall. And at the back of the main viewpoint you find an inscription saying to return when "your story ends". It doesn't mean the storyline, but 100 % of completion the game (check the proper chapter of this guide for more information). When you make it, return there at night (at 3 am to be precise) to see UFO over the mountain in the storm. The painting may suggest also some hidden room inside the mountain - maybe it will be a part of DLC.
What is more, this mountain contains many more interesting views and situations. You can run into, for example, a random event when you hear about missing motorcyclists in the radio and then wandering around you hear their voices.
There are also Altruist cultists on the mountain mentioned above. It's a cannibal village which is cut off from the civilization. Trevor may transport there hitchhikers (there is an achievement for that). What is more, when you get there four people, cultists will try to capture Trevor, who will have to fight his way out of there. In this village you can find, among others, RPG, Assault Rifle, baseball cap and a lot of money. When you pick up a hitchhiker, its location shows up - it's the easiest way to get there. You can also place there your private Point of Interest to not forget about it.
Another "thing to check" is also the Internet. Normally, browsing ends on checking shares and your bank account. But the game offers also many internet pages full of specific humor, which is typical for GTA series. You can find, among others, a perfect name for your children (www.yournewbabysname.com) and get to know more about your ancestors (www.yourdeadfamily.com). Click links, ads and use browser to visit more and more pages. You find about hundred pages so pick up phone and start surfing.