The Kortz Center- north west of Los Santos is enterable. If you look at the paper map that came with your game (the side which has the magnified version of Los Santos), it's in B2. From Los Santos, follow N Rockford Drive going north, just after you left the city, you'll see the entrance at your left side. From the entrance, it's still quite a long drive to the Kortz Center mall. It's a pretty big mall or so it seems, looks like a fun place to have multiplayer matches. While the mall itself is mostly outdoors, you have to enter through the glass doors and go through a space with a couple ATM machines. The mall features terraces, lot's of stairs and bridges and roofs you can climb onto. Behind that mall is a hedge maze.
Playboy Mansion- (technically not an interior, but a cool little eater egg) On american way (richman) , located in B3 on the zoomed in version of los santos there is a private drive leading to a near replica of the playboy mantion. Visit it at night for a lively party of peds and even topless chicks
Villa construction site- Dunstable lane. Northwest of that tall tower in the north Rockford Hills.
Police Station- Near the north east side of Los Santos, right next to the impound lot.
Huge building construction site- in between Del Perro Freeway and the hairdresser in the south of Vinewood. When you zoom in on the map, the area pointer says 'ALTA'.
Huge construction site #2- It's the incredibly tall tower that's under construction. It's located at the Adam's Apple Blvd just north of the Olympic freeway in the DownTown area. If you go up in the lower building, you can get into the tower through a bridge structure on the roof. Unfortunately, you cannot get higher up into the tower from that point, because all the stairs are locked behind fences. (Note) there may be another way in. Good place for multiplayer matches.
Land Act Dam- in Tataviam Mountains has a room and small passageway (downstairs) you can enter. There's also a parachute in one of the rooms if I'm not mistaken.
Lumber Yard- South of Paleto Bay in the Paleto Forest, it's right next to the train track. It has some Storage space you can enter.
Sewer system- Take the Del Perro Freeway in Los Santos to the east. When you go under the Alta Street bridge, there's a hole in the wall on the right side. There's a huge cavern going downwards, leading into the sewer system. This place is huge!
Abandoned Motel- Sandy Shores, Panorama Drive. It's quite big and some of the rooms don't have doors so you can enter them. Everything is trashed and demolished. The roof has collapsed and you can use the rubble to go outside onto the roof.
Darnell Bros Garment Factory- This is Lesters planning hideout. It's located in between San Andreas Ave and Vespucci Blvd on the east side of the canal. (F3 on the Los Santos map). It has 2 stories and several rooms.(Enterable after Lester introduces you to it)
Los Santos Marine- Elysian Island (D5 on the map): it's basically a large hangar
Large Garage- Elysian Island (D5 on the map)
Vinewood Racetrack- in East Vinewood. It's not much, but you can climb the fence and go inside to the seating area and the race track.
Train track tunnel- From the Ammunation with shooting range at the Dry Dock street in east Los Santos, go down to the water stream that goes from north to south. There's a small tunnel leading east, straight into the long train track tunnel.
Bugstars Pest Control company- Signal Street, Elysian Island in the south of Los Santos (E6 on the LS map). It's basically a large garage, it has company vans inside and a health pack.
Recycling Company- Alta Street (D4 on the LS map), just northwest of the Maze Bank Arena. It has a couple of accessible dilapidated storage spaces. The office/factory itself is closed, it's only accessible in a mission.
Bar- Located in sandy shores
24/7 Supermarket Locations
Chumash, Barbareno Road (along the westcoast).
Tequila-la- club, enter in the back door. ( )
Hospital- is enterable in a side mission of trevor's -
The city morgue- is enterable in a mission as michael
Lifeinvader offices- enterable for an assassination mission
(Thanks to TheFlush, TheRealBars these finds so far)
***Lots of marked interiors- http://i.imgur.com/9TJqlbt.jpg