The Altruist Cult - The Altruist Cult has its hangout in Mount Chilliad. It is a city of cannibals away from any civilization. Trevor may deliver there the hitchhikers that he encounters (it is rewarded with and achievement). Additionally, after the delivering of four hitchhikers, the denizens will also want to capture Trevor, who will have to use force to get out of there. On the premises of the hangout, you can find an RPG, an Assault Rifle, a baseball bat and over $100.000, among others. The easiest way to get there is by hiving a ride to a hitchhiker, while playing as Trevor. The location will then get marked on the map.
Mount Chilliad - the highest peak in the game. The easiest way to get there is by taking a monorail. This is a grand place, in itself, but you can also find a lot of surprises here. At the very peak of the mountain, you can find a parachute and two dirtbikes. You can try to ride down the mountain, jump off it or both, bu releasing the parachute when you are falling on the bike. Apart from that, at the back of the main scenic point, there is a mural, which reads that you should come back after "your story is complete". This means that you need to complete the game in 100%. Once you do that and return here during storm, at night, you will notice a UFO hovering above the mountain. The mural itself can also suggest that there may be some secret chambers hidden within the mount.
Fort Zancudo - A mysterious military base. You will find yourself around the fort for several times, while performing various missions. Still, not necessarily inside, but it is worth visiting anyways. You can obtain there, e.g. the Rhino tank, Lazer P-996 jet, Buzzard helicopter or even the formidable Titan. Entering the base, however, means adding four stars to your wanted level. Unfortunately, you cannot sneak into the base and, the longer you are going to stay there, the more difficult it will be for you, due to the increasing numbers of soldiers and heavy equipment. Still, you can have some fun inside the fort. You can enter the main building with the tower and make it onto its top. Apart from the interesting training facility and the view over the entire base, you can also find here lots of weapons scattered around, including a minigun.
Humane Labs and Research Company - Another top secret compound and chemical labs. Like in the case of Fort Zacundo, you will receive four stars for intruding into the premises. You visit the labs while completing a mission but, from inside. To see it from the outside, you need to go there on your own. What is interesting, you can use a cab for that, which will take you directly to the main entrance. Still, you are getting four stars straight away.
The Prison - Bolingbroke Penitentiary located directly to the South of Trevor's hangar in Sandy Shores, is another place that you can visit for adventure. Any attempt to break into the prison ends up in stars, but it is completely safe to drive around the fence. Near the prison, you can also find inmates that managed to escape. They will be posing as hitchhikers but, in fact, they will try to threaten you with weapon trying to steal your car. It is easy to recognize them by orange uniforms.
Ron Alternates Wind Farm - A wind farm full of turbines. Planes and helicopters caught between the blades explode in the air, thus providing a lot of fireworks and fun. You can, e.g. try to lure into this area the police choppers that are chasing you.
Galileo Observatory - an observatory modeled after the Griffith Observatory in Los Angeles.
VINEWOOD sign - a huge VINEWOOD sign modeled, of course, after the famous HOLLYWOOD sign. You can climb any of the letters to admire the city's panorama.
Vinewood Bowl - The amphitheater modeled after the famous Hollywood Bowl
Racetrack - A huge racetrack in Vinewood
National Office of Security Enforcement - In the east coast, you will find the NOOSE headquarters. It is a government complex. Unlike Fort Zancudo and HLRC entering here does not result in police pursuit. Among the thing that you can find here, there is an armed Buzzard chopper on the rooftop. To get up there, you will need to climb up the ladder at the pback, on the right of the main entrance and make it onto the building on the left over the rooftops.
The funfair - you can ride on the rail or take a ride on the Ferris wheel to take a look at the overwhelming vastness of Los Santos.
Maze Bank Tower - The tallest building in LS, modeled after US Bank Tower in Los Angeles. A great scenic point, with a usable Helipad on the rooftop (not purchase-able). Located in Pillbox Hill, in the city center. If you have problems finding it, simply look up.
Maze Bank Arena - The headquarters of the local basketball team - Los Santos Panic (a parody of Los Angeles Lakers) and the location of shooting Fame or Shame.
The Airport - Also the main airport in Los Santos is worth visiting. You can obtain passenger planes, like Luxor, here. Apart from that, you can also steal the big planes modeled after Boeing 747, which cannot be bought. Unfortunately, you cannot store them, because they are too sizeable to fit into any hangar. They also lack their own entrance animations. To get onto the plane, simply press the enter/exit button and you will automatically be placed in the cockpit. Remember, if you do not own a hangar at the airport, entering there will result in a three-star pursuit.